1. technology and computing
2. software
3. graphics software

DRAFT CHAPTER 8: Equations of Lines and Planes
Errors will be corrected before printing. Final book will be available August 2008.
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Chapter 8
EQUATIONS OF LINES AND PLANES
In this chapter, you will work with vector concepts learned in the preceding chapters
and use them to develop equations for lines and planes. We begin with lines in R2
and then move to R3, where lines are once again considered along with planes.
The determination of equations for lines and planes helps provide the basis for an
understanding of geometry in R3. These concepts also provide the foundation for
the solution of systems of linear equations which result from intersections of lines
and planes, which are considered in Chapter 9.
CHAPTER EXPECTATIONS
In this chapter, you will
• determine the vector and parametric equations of a line in two-space, Section 8.1
• make connections between Cartesian, vector, and parametric equations of a line
in two-space, Section 8.2
• determine the vector, parametric, and symmetric equations of a line in threespace, Section 8.3
• determine the vector, parametric, and Cartesian equations of a plane,
Sections 8.4, 8.5
• determine some geometric properties of a plane, Section 8.5
• determine the equation of a plane in Cartesian, vector, or parametric form,
given another form, Section 8.5
• sketch a plane in three-space, Section 8.6
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Review of Prerequisite Skills
In this chapter, we will develop the equation of a line in two- and threedimensional space and the equation of a plane in three-dimensional space.
You will find it helpful to review the following concepts:
• geometric and algebraic vectors
• the dot product
• the cross product
• plotting points and vectors in three-space
We will begin the chapter by examining equations of lines. Lines are not vectors,
but vectors are used to describe lines. The table below shows their similarities
and differences.
Lines
Vectors
Lines are bi-directional. A line defines
a direction, but there is nothing to
distinguish forward from backwards.
Vectors are unidirectional. A vector defines a
direction with a clear distinction between forward
and backwards.
A line is infinite in extent in both
directions. A line segment has a finite
length.
Vectors have a finite magnitude.
Lines and line segments have a definite
location. The opposite sides of a
parallelogram are two different line
segments.
A vector has no fixed location. The opposite sides
of a parallelogram are described by the same
vector.
Two lines are the same when they have
the same direction and same location.
These lines are said to be coincident.
Two vectors are the same when they have the
same direction and the same magnitude. These
vectors are said to be equal.
Exercise
1. Determine a single vector equivalent to each of the following expressions:
a. 13, 22, 1 2 2 11, 7, 25 2
b. 512, 23, 242 1 311, 1, 272
a. A11, 232 B14, 2 2 C128, 218 2
c. A11, 2, 1 2 B14, 7, 0 2 C17, 12, 21 2
2. Determine if the following sets of points are collinear.
b. J124, 32 K14, 5 2 L10, 4 2
2
REVIEW OF PREREQUISITE SKILLS
d. R11, 2, 23 2 S14, 1, 32 T12, 4, 02
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3. Determine if ^ABC is a right-angled triangle, given A11, 6, 222 , B12, 5, 32 ,
and C15, 3, 22 .
!
!
4. Given u 5 1t, 21, 32 and v 5 12, t, 262 , for what values of t are the vectors
perpendicular?
5. State a vector perpendicular to each of the following:
!
!
!
a. a 5 11, 232
b. b 5 16, 25 2
c. c 127, 24, 02
6. Calculate the area of the parallelogram formed by the vectors 14, 10, 9 2 and
13, 1, 22 2.
7. Use the cross product to determine a vector perpendicular to each of the
following pairs of vectors. Check your answer using the dot product.
!
!
a. a 5 12, 1, 24 2
b 5 13, 25, 22 2
!
!
b. a 5 121, 22, 02
b 5 122, 21, 0 2
8. For each of the following, draw the x-axis, y-axis, and z-axis and accurately
draw the position vectors.
a. A11, 2, 3 2
b. B11, 2, 232
c. C11, 22, 3 2
d. D121, 2, 3 2
9. Determine the position vector that passes through each pair of points.
a. 14, 8 2 and 123, 5 2
b. 127, 262 and 13, 8 2
c. 11, 2, 4 2 and 13, 26, 92
d. 14, 0, 24 2 and 10, 5, 0 2
10. State the vector that is opposite to each of the vectors you found in question 9.
11. For each of the following linear equations
a. determine its slope and y-intercept
b. sketch its graph
i. y 5 22x 2 5
iii. 3x 2 5y 1 1 5 0
ii. 4x 2 8y 5 8
iv. 5x 5 5y 2 15
12. State a vector that is collinear and in the same direction of each of the
following.
!
!
!
!
!
!
a. 14, 7 2
b. 125, 4, 3 2
c. 2i 1 6j 2 4k
d. 25i 1 8j 1 2k
!
!
!
!
!
13. If u 5 14, 29, 21 2 and v 5 4i 2 2j 1 k determine each of the following.
! !
!
!
a. u # v
d. u 3 v
! !
!
!
b. 2v # u
e. v 3 u
!
!
!
!
!
!
!
!
c. 1u 1 v 2 # 1u 2 v 2
f. 12u 1 v 2 3 1u 2 2v 2
14. Both the dot product and the cross product are ways to multiply two vectors.
Explain how these products differ.
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Investigate
CAREER LINK
CHAPTER 8: COMPUTER PROGRAMMING WITH VECTORS
Computer programmers use vectors for a variety of graphics applications. Any
time two- and three-dimensional images are designed, they are represented in
the form of vectors. Vectors allow the programmer to easily move the figure to
any new location on the screen. If the figure were expressed point by point
using coordinate geometry, each and every point would have to be recalculated
each time the figure needed to be moved. By using vectors drawn from an
anchor point to draw the figure, only the coordinates of the anchor point need
to be recalculated on the screen to move the entire figure. This method is used
in many different types of software, including games, flight simulators, drafting
and architecture tools, and visual design tools.
Case Study – Breakout
Breakout is a classic video game, and a prime example of using vectors in
computer graphics. A paddle is used to bounce a ball into a section of bricks to
slowly break down a wall. Each time the ball hits the paddle, it bounces off at
an angle to the paddle. The path the ball takes from the paddle can be
described by a vector, which is dependent upon the angle and speed of the ball.
If the wall were not in the path of the ball, the ball would continue along its
path at that speed until it “fell” off of the screen.
DISCUSSION QUESTIONS
1. The coordinate plane represents the screen in the! game “Breakout.” The ball
y
7
r
6
5
4
3
2
1
x
–3 –2 –1
paddle
4
0
CAREER LINK
1
is traveling towards the paddle along the vector r . Find the vector and
!
parametric equations of the line determined by the vector r . Draw a
direction vector for the line.
2. Find where the line crosses the x-axis to show where the paddle must move
in order to bounce the ball back.
3. Since the angle of entry for the ball is 45º,
the ball will bounce off the
!
!
!
paddle along a path perpendicular to r . Draw a vector s perpendicular to r
!
that passes through the origin (0, 0). Then draw a line parallel to vector s
!
that passes through the point where r crosses the x-axis.
2
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Section 8.1—Vector and Parametric Equations
of a Line in R2
In this section, we begin with a discussion on how to find the vector and
parametric equations of a line in R2. In order to find vector and parametric
equations for a line, we must be given either two distinct points or one point and a
vector that defines the direction of the line. In either situation, a direction vector
for the line is necessary. A direction vector is defined to be a nonzero vector
!
!
m 5 1a, b2 parallel (collinear) to the given line. The direction vector m 5 1a, b 2
is represented bya vector with its tail at the origin and its head at the point 1a, b 2.
The x and y components of this direction vector are called its direction numbers.
In thecase of the vector 1a, b 2 , the direction numbers are a and b.
EXAMPLE 1
Representing lines using vectors
!
a. A line passing through P14, 3 2 has m 5 127, 1 2 as its direction vector. Draw a
sketch of this line.
1
3 1
b. A line passes through the points AQ2, 23R and BQ4, 2R. Determine a direction
vector for this line and write it using integer components.
Solution
!
a. The vector m 5 127, 1 2 is a direction vector for the line and is shown on the
!
graph. The required line is parallel to m and passes through P14, 3 2 . This line
!
is drawn through P14, 3 2 parallel to m .
6
y
4
m = (– 7, 1)
P(4, 3)
Required Line with direction
x vector (– 7, 1)
2
–6 –4 –2 0
–2
2
4
6
–4
–6
b. In determining a direction vector for the line through AQ2, 23R and BQ4, 2R,
!
!
we determine a vector equivalent to either AB or BA .
!
!
3
1 7
1 1
1 7
AB 5 a 2 , 2 123 2 b 5 a , b or BA 5 a 2 , 2 b
4
2 2
4 2
4 2
1
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Both these vectors can be multiplied by 4 in order to ensure both direction
!
!
numbers are integers. As a result, either m 5 11, 142 or m 5 121, 214 2 are the
best choices for a direction vector. When we determine a direction vector in this
case, any scalar multiple of this vector of the form t11, 14 2 is correct, provided
that t Þ 0. If t 5 0, 10, 0 2 would be the direction vector, meaning that the line
would not have a defined direction.
Expressing the Equations of Lines Using Vectors
In general, we would like to determine the equation of a line if we have a
direction for the line and a point on it. In the following diagram,
the given point
!
!
P0 1x0, y0 2 is on the line L and is associated with vector OP 0, designated as r0 .
!
!
The direction of the line is given by m 5 1a, b2 , where tm , t [ R is any vector
!
!
collinear with m . P1x, y2 represents a general point on the line where OP is the
vector associated with this point.
y
r = r0 + tm , t [R
P0 (x0 , y0) tm
r0
L
P(x, y)
r
m = (a, b)
O
x
To find the vector equation of line L, the triangle law of addition is used.
!
!
!
In triangle OP0P, OP 5 OP0 1 P0P .
! !
!
!
!
!
Since r 5 OP , r0 5 OP0 , and tm 5 P0P , the vector equation of the line is
!
!
!
written as r 5 r0 1 tm , t [ R.
When writing an equation for a line using vectors, the vector form of the line is
sometimes modified and put in parametric form. The parametric equations for a
line come directly from its vector equation. The changing of an equation for a line
from vector to parametric form is shown as follows.
!
!
!
The general vector equation for a line is r 5 r0 1 tm , t [ R.
In component form, this is written as 1x, y2 5 1x 0, y0 2 1 t1a, b2 , t [ R.
Expanding the right side, 1x, y2 5 1x 0, y0 2 1 1ta, tb2 5 1x 0 1 ta, y0 1 tb2 , t [ R.
If we equate the respective x and y components, the required parametric form is
x 5 x0 1 ta and y 5 y0 1 tb, t [ R.
Vector and Parametric Equations of a Line in R2
!
!
!
Vector Equation: r 5 r0 1 tm , t [ R
Parametric Equations: x 5 x0 1 ta, y 5 y0 1 tb, t [ R
6
8.1
VECTOR AND PARAMETRIC EQUATIONS OF A LINE IN R 2
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In either vector or parametric form, t is called a parameter. This means that t can be
replaced by any real number in order to obtain the coordinates of points on the line.
EXAMPLE 2
Reasoning about the vector and parametric equations of a line
a. Determine the vector and parametric equations of a line passing through the
!
point A11, 42 with direction vector m 5 123, 3 2 .
b. Draw a sketch of the line and determine the coordinates of four points on the
line.
c. Is either of the points Q1221, 232 or R1229, 342 on this line?
Solution
!
!
!
a. Since A(1, 4) is on the line, OP 0 5 r 0 5 11, 42 and m 5 123, 3 2 , and the
!
vector equation is r 5 11, 42 1 t123, 3 2 , t [ R. The parametric equations are
x 5 1 2 3t, y 5 4 1 3t, t [ R.
!
It is also possible to use other scalar multiples of m 5 123, 3 2 as a direction
vector, such as 121, 1 2 which gives the respective vector and parametric
!
equations r 5 11, 4 2 1 s121, 1 2 , s [ R and x 5 1 2 s, y 5 4 1 s, s [ R.
The vector 121, 12 has been chosen as our direction vector for the sake of
simplicity. Note that we have written the second equation with parameter
s to avoid confusion between the two lines. Although the two equations,
!
!
r 5 11, 42 1 t123, 32 , t [ R and r 5 11, 4 2 1 s121, 12 , s [ R, appear with
different parameters, the lines they represent are identical.
b. In determining the coordinates of points on the line, the parametric equations
x 5 1 2 s, y 5 4 1 s, s [ R were used with s chosen to be 0, 1, 21 and 26.
To find the coordinates of a particular point, such as D, s 5 26 was substituted
into the parametric equations and x 5 1 2 126 2 5 7, y 5 4 1 126 2 5 22.
The required point is D17, 222 . The coordinates of the other points are
determined in the same way using the other values of s.
y
6 s = 1, A(0, 5)
s = 0, B(1, 4)
4
s = –1, C(2, 3)
2
m = (– 1, 1)
–2 0
–2
x
2
4
6
8
s = –6, D(7, –2)
–4
–6
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c. If the point Q1221, 232 lies on the line, then there must be consistency with
the parameter s. We substitute this point into the parametric equations to check
for the required consistency. Substituting gives 221 5 1 2 s and 23 5 4 1 s.
In the first equation, s 5 22, and in the second, s 5 19. Since these values are
inconsistent, the point Q is not on the line.
If the point R1229, 342 is on the line, then 229 5 1 2 s and 34 5 4 1 s,
s 5 30 for both equations.
Since each of these equations has the same solution, s 5 30, we conclude that
R1229, 342 is on the line.
Sometimes the equation of the line must be found when two points are given.
This is shown in the following example.
EXAMPLE 3
Connecting vector and parametric equations with two points on a line
a. Determine vector and parametric equations for the line containing E121, 52
and F16, 112.
b. What are the coordinates of the point where this line crosses the x-axis?
!
c. Can the equation r 5 1215, 27 2 1 t Q14
, 4R, t [ R also represent the line
3
containing E and F?
Solution
a. A direction vector for the line containing E and F is
!
m 5 16 2 1212, 11 2 5 2 5 17, 6 2 . A vector equation for the line is
!
r 5 121, 52 1 s17, 62 , s [ R, and its parametric equations are
x 5 21 1 7s, y 5 5 1 6s, s [ R.
The equation given for this line is not unique. This is because there are an
infinite number of choices for the direction vector, and any point on the line
could have been used. In writing a second equation for the line, the parametric
equations x 5 6 1 7s, y 5 11 1 6s, s [ R would also have been correct
because 16, 11 2 is on the line and the direction vector is 17, 6 2 .
b. The line intersects the x-axis at a point with coordinates of the form 1a, 0 2 . At
25
the point of intersection, y 5 0 and so, 5 1 6s 5 0, s 5 6 . Therefore,
a 5 21 1 7s
5 21 1 7Q25
R 5 241
, and the line intersects the x-axis at the point Q241
, 0R.
6
6
6
8
8.1
VECTOR AND PARAMETRIC EQUATIONS OF A LINE IN R 2
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c. If this equation represents the same line as the equation in a., it is necessary
that the two lines have the same direction and contain the same set of points.
!
14
14
The line r 5 1215, 27 2 1 t Q 3 , 4R, t [ R has Q 3 , 4R as its direction vector. The
two lines will have the same direction vectors because 2Q 3 , 4R 5 17, 6 2 . The
3 14
two lines have the same direction, and if these lines have a point in common,
then the equations represent the same line. The easiest approach is to substitute
1215, 272 into the first equation to see if this point is on the line. Substituting
gives 1215, 27 2 5 121, 5 2 1 s17, 62 or 215 5 21 1 7s and 27 5 5 1 6s.
Since the solution to each of these equations is s 5 22, the point 1215, 27 2 is
on the line, and the two equations represent the same line.
In the next example, vector properties will be used to determine equations for
lines that involve perpendicularity.
EXAMPLE 4
Selecting a strategy to determine the vector equation of a
perpendicular line
Determine a vector equation for the line that is perpendicular to
!
r 5 14, 12 1 s123, 22 , s [ R and which passes through the point P16, 52 .
Solution
!
The direction vector of the given line is m 5 123, 2 2 and this line is drawn
through 14, 12, as shown in red in the diagram. A sketch of the required line,
passing through 16, 5 2 and perpendicular to the given line, is also drawn in blue.
6
y
P(6, 5)
4
(– 3, 2)
2
(4, 1)
m = (– 3, 2)
x
–4 –2 0
–2
2
4
6
8
r = (4, 1) + s(–3, 2)
–4
–6
!
Let the direction vector for the required blue line be v 5 1a, b 2 . Since the direction
! !
vector of the given line is perpendicular to that of the required line, v # m 5 0.
Therefore, 1a, b2 # 123, 2 2 5 0 or 23a 1 2b 5 0.
NEL
The simplest integer values for a and b satisfying this equation are a 5 2 and
b 5 3. This gives the direction vector 12, 3 2 and the required vector equation for
!
the perpendicular line is r 5 16, 52 1 t12, 32 , t [ R.
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In this section, the vector and parametric equations of a line in R2 were examined.
In Section 8.3, this discussion will be extended to R3 where many of the ideas
seen in this section apply to lines in three-space.
The following investigation is designed to aid in understanding the concept of
parameter when dealing with either the vector or parametric equations of a line.
INVESTIGATION
A. In the following, two lines with equations are given. These lines are
!
!
L 1: r 5 t10, 12, t [ R and L 2 : r 5 p11, 0 2 , p [ R.
i. On graph paper, draw each of the given lines, making certain to clearly
show a direction vector for each line.
ii. Describe geometrically what each of the two equations represent.
iii. Give a vector equation and corresponding parametric equations for each of
the following.
• the line parallel to the x-axis passing through P12, 4 2
• the line parallel to the y-axis passing through Q122, 212
iv. Draw a sketch of each of the following, using your own axes.
L 3: x 5 23, y 5 1 1 s, s [ R and L4: x 5 4 1 t, y 5 1, t [ R
v. By examining parametric equations for a line, how is it possible to
determine by inspection whether a line is parallel to either the x-axis or
y-axis?
vi. Write an equation for a line in both vector and parametric form that is
parallel to the x-axis.
vii. Write an equation for a line in both vector and parametric form that is
parallel to the y-axis.
!
B. i. Draw a sketch of the line L : r 5 123, 0 2 1 s12, 21 2 , s [ R on graph paper.
ii. On the set of axes used for 2. a., sketch each of the following.
!
• L 1 : r 5 122, 1 2 1 s12, 21 2 , s [ R
!
• L 2 : r 5 123, 1 2 1 s12, 21 2 , s [ R
!
• L 3 : r 5 12, 21 2 1 s12, 21 2 , s [ R
!
• L 4 : r 5 14, 22 1 s12, 21 2 , s [ R
!
!
If you are given the equation r 5 r0 1 s12, 21 2 , s [ R, what is the
!
mathematical effect of changing the value of r0 ?
!
iii. For the line L 1 : r 5 122, 1 2 1 s12, 21 2 , s [ R, show that each of the
following points are on this line by finding corresponding values of
s: 14, 222 , 124, 2 2 , 1198, 299 2 , and 12202, 101 2 .
!
!
!
iv. Which part of the equation r 5 r0 1 tm , t [ R indicates that there are an
infinite number of points on this line? Explain your answer.
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8.1
VECTOR AND PARAMETRIC EQUATIONS OF A LINE IN R 2
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IN SUMMARY
Key Idea
!
!
!
!
• The vector equation of a line in R2 is r 5 r0 1 tm , t [ R where m 5 1a, b 2 is
the direction vector and r0 is the vector from the origin to the point r0.
• The parametric form of the equation of a line is x 5 x0 1 ta and
y 5 y0 1 tb, t [ R.
Need to Know
• In both the vector and parametric equations, t is a parameter. Every real
number for t generates a different point that lies on the line.
Exercise 8.1
PART A
!
1. A vector equation is given as r 5 Q12, 23
R 1 sQ13, 16R, s [ R. Explain why each
4
of the following are acceptable direction vectors for this line:
!
!
!
m 5 122, 21 2 , m 5 12, 12 , and m 5 Q27, 17R.
2. Parametric equations of a line are x 5 1 1 3t and y 5 5 2 2t, t [ R.
a. Write the coordinates of three points on this line.
b. Show that the point P1214, 152 lies on the given line by determining the
parameter value of t corresponding to this point.
3. Identify the direction vector and a point on each of the following lines.
!
a. r 5 13, 4 2 1 t12, 1 2 , t [ R
b. x 5 1 1 2t, y 5 3 2 7t, t [ R
!
c. r 5 14, 1 1 2t 2 , t [ R
d. x 5 25t, y 5 6, t [ R
PART B
4. A line passes through the points A12, 1 2 and B123, 52 . Write two different
vector equations for this line.
5. A line is defined by the parametric equations x 5 22 2 t and
y 5 4 1 2t, t [ R.
a. Does R129, 182 lie on this line? Explain.
b. Write a vector equation of this line using the given parametric equations.
c. Write a second vector equation for this line different from the one you
wrote in 5.b.
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!
6. a. If the equation of a line is r 5 s13, 4 2 , s [ R, name the coordinates of
three points on this line.
C
b. Write a vector equation, different from the one given, for a line that passes
through the origin.
!
c. Describe the line with equation r 5 19, 122 1 t13, 42 , t [ R.
!
7. A line has r 5 Q13, 17R 1 p122, 3 2 , p [ R as its vector equation. A student
decides to “simplify” this equation by clearing fractions, and multiplies the
!
vector Q13, 17R by 21 and obtains r 5 17, 32 1 p122, 3 2 , p [ R as a “correct”
form of the line. Explain why multiplying a point in this way is incorrect.
8. A line passes through the points Q10, 7 2 and R10, 9 2 .
a. Sketch this line.
b. Find vector and parametric equations for this line.
K
9. A line passes through the points M14, 5 2 and N19, 5 2 .
a. Draw a sketch of this line.
b. Find vector and parametric equations for this line.
!
10. For the line L : r 5 11, 25 2 1 s13, 5 2 , s [ R, determine the following:
a. an equation for the line perpendicular to L passing through P12, 0 2
b. the point at which the line in 10. a. intersects the y-axis
A
T
11. The parametric equations of a line are given as
x 5 210 2 2s, y 5 8 1 s, s [ R. This line crosses the x-axis at the point
with coordinates A1a, 0 2 and crosses the y-axis at the point with coordinates
B10, b2. If O represents the origin, determine the area of the triangle AOB.
!
12. A line has r 5 11, 22 1 s122, 3 2 , s [ R as its vector equation. On this line,
the points A, B, C, and D correspond to parametric values s 5 0, 1, 2, and 3,
respectively. Show that each of the following is true.
!
!
!
!
! 2 !
a. AC 5 2AB
b. AD 5 3AB
c. AC 5 3 AD
PART C
13. The line L has x 5 2 1 t, y 5 9 1 t, t [ R as its parametric equations. If
L intersects the circle with equation x 2 1 y 2 5 169 at points A and B,
determine the following:
a. the coordinates of the points A and B
b. the length of the chord AB
14. Are the lines 2x 2 3y 1 15 5 0 and 1x, y2 5 11, 62 1 t16, 42 parallel?
Explain.
12
8.1
VECTOR AND PARAMETRIC EQUATIONS OF A LINE IN R 2
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Section 8.2—Cartesian Equation of a Line
In the previous section, we discussed the vector and parametric equations of
lines in R2. In this section, we will show how lines of the form y 5 mx 1 b
(slope–y-intercept form) and Ax 1 By 1 C 5 0 (Cartesian equation of a line,
also called a scalar equation of a line) are related to the vector and parametric
forms of the line.
The Relationship Between Vector and Scalar Equations of Lines in R 2
The direction, or inclination of a line, can be described in two ways—either by its
slope, or by a direction vector. The slope of the line joining two points A1x0, y0 2
and B1x1, y1 2 is given by the formula m 5 run 5 x1 2 x0. It is also possible to
1
0
describe
the
direction
of
a
line
using
the
vector
defined
by the two points A and B,
!
!
AB 5 m 5 1x1 2 x0, y1 2 y0 2 . This direction vector is equivalent to a vector with
its tail at the origin and its head at C1x1 2 x0, y1 2 y0 2 and is shown in the diagram
below.
y 2y
rise
y
B(x1 , y1)
y1 – y 0
A(x0 , y0) x1 – x0
C(x1 – x0 , y1 – y0 )
x
O(0, 0)
Direction Vectors and Slope
In the diagram, a line segment AC with slope m 5 ba is shown
!
with a run of a and a rise of b. The vector m 5 1a, b2 is used
to describe the direction of this line or any line parallel to it
with no restriction on the direction numbers a and b. In
practice, a and b can be any two real numbers when describing
a direction vector. When the direction vector of a line is
!
m 5 1a, b2 this corresponds to a slope of m 5 ba except
when a 5 0.
y
m= b
a
A
a
C
b
B
O(0, 0)
a
b m = (a, b)
x
In the following example, it will be shown how to take a line in slope–y-intercept
form and convert it to vector and parametric form.
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Representing the Cartesian equation of a line in vector and
parametric form
3
Determine the equivalent vector and parametric equations for the line y 5 4 x 1 2.
Solution
3
In the diagram below, the line y 5 4 x 1 2 is drawn. This line passes through
!
3
10, 2 2 with a slope of m 5 4 and as a result has a direction vector m 5 14, 32. A
!
vector equation for this line is r 5 10, 22 1 t14, 3 2, t [ R with parametric equations
x 5 4t, y 5 2 1 3t, t [ R.
6
y
y = 43 x + 2
4
(0, 2)
2
m = (4, 3)
x
0
–6 –4 –2
2 4 6
–2 O(0, 0)
In the next example, we will show the conversion of a line in vector form to one
in slope–y-intercept form.
EXAMPLE 2
Representing a vector equation of a line in Cartesian form
!
For the line with equation r 5 13, 26 2 1 s121, 24 2 , s [ R determine the
equivalent slope–y-intercept form.
Solution
Method 1:
!
24
The direction vector of this line is m 5 121, 242 with slope m 5 21 5 4.
This line contains the point 13, 26 2. If P1x, y2 represents a general point on this
line, then we can use slope-point form to determine the required equation.
y 2 126 2
Thus, x 2 3
54
41x 2 32 5 y 1 6
4x 2 12 5 y 1 6
4x 2 18 5 y
The required equation for the line is y 5 4x 2 18 in slope–y-intercept form.
14
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Method 2:
We start by writing the given line in parametric form,
i.e., 1x, y2 5 13, 26 2 1 s121, 24 2 or 1x, y2 5 13 2 s, 26 2 4s2. This gives the
parametric equations, x 5 3 2 s and y 5 26 2 4s. In order to find the required
x23
y16
equation, we solve for s in each component. Thus, s 5 21 and s 5 24 . Since
x23
y16
these equations for s are equal, 21 5 24
241x 2 3 2
5y16
21
y 1 6 5 41x 2 3 2
y 5 4x 2 18.
Therefore the required equation is y 5 4x 2 18, which is the same answer as we
obtained using Method 1.
In the example that follows, we examine the situation where the direction vector
!
of the line is of the form, m 5 10, b2 .
EXAMPLE 3
Reasoning about equations of vertical lines
Determine the Cartesian form of the line having the equation
!
r 5 11, 4 2 1 s10, 2 2, s [ R.
Solution
The given line passes through the point 11, 4 2 with direction vector 10, 2 2, as
shown in the diagram below.
y
6
4
2
m = (0, 2)
–4 –2 0
–2
A(1, 4)
B(1, 0) x
2 4
–4
It is not possible, in this case, to calculate the slope because the line has direction
2
vector 10, 22, meaning its slope would be 0, which is undefined. Since the line is
parallel to the y-axis, it must have the form x 5 a where 1a, 0 2 is the point where
the line crosses the x-axis. The equation of this line is x 5 1.
Developing the Cartesian Equation From A Direction Vector
In addition to making the connection between lines in either slope–y-intercept form
or Cartesian form with those in vector form, we would also like to consider how
direction vectors can be used to obtain the equations of lines in Cartesian form.
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In the following diagram, the line L represents a general line in R2. A line has
been drawn from the origin, perpendicular to L. This perpendicular line is called
the normal axis for the line and is the only line that can be drawn from the origin
perpendicular to the given line. If the origin is joined to any point on the normal
axis, other than itself, the vector formed is described as a normal to the given line.
Since there are an infinite number of points on the normal axis, this is a way of
saying that any line in R2 has an infinite number of normals, none of which is the
zero vector. A general point on the normal axis is given the coordinates N1A, B2
!
!
and so a normal vector, denoted by n , is the vector n , 5 1A, B2.
normal axis
y
L
n = (A, B)
O(0,0)
x
The important property of the normal vector is that it is perpendicular to any
vector on the given line. This property of normal vectors is what allows us to
derive the Cartesian equation of the line.
!
In the following diagram, the line L is drawn, along with a normal n 5 1A, B2
to L. The point P1x, y2 represents any point on the line, and the point P1x0, y0 2
represents a given point on the line.
n = (A, B)
normal axis
y
L
P (x, y)
P0 (x0, y0)
m = (x – x0, y – y0)
x
O(0,0)
!
In deriving the Cartesian equation !for this line, we first determine P0P . In
coordinate form, this vector is P0P 5 1x 2 x 0, y 2 y0 2, which represents a
direction vector for the line. In the diagram, this vector
has been shown as
!
!
!
m 5 1x 2 x0, y 2 y0 2 . Since the vectors n and P0P are perpendicular to each
!
!
other, n # P0P 5 0.
16
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Thus,
1A, B2 # 1x 2 x0, y 2 y0 2 5 0
Multiplying out, Ax 2 Ax0 1 By 2 By0 5 0
Rearranging,
Ax 1 By 2 Ax0 2 By0 5 0
Since the point P0 1x0, y0 2 is a point whose coordinates are known, as is
!
n 5 1A, B2, we substitute C for the quantity 2Ax0 2 By0 to obtain
Ax 1 By 1 C 5 0 as the Cartesian equation of the line.
Cartesian Equation of a Line in R2
In R2, the Cartesian equation of a line (or scalar equation) is given by
!
Ax 1 By 1 C 5 0, where a normal to this line is n 5 1A, B2 . A normal to this
line is a vector drawn from the origin perpendicular to the given line to the
point N1A, B2 .
EXAMPLE 4
Connecting the Cartesian equation of a line to its normal
Determine the Cartesian equation of the line passing through A14, 22 2 which has
!
n 5 15, 32 as its normal.
Solution
!
The required line is sketched by first drawing the normal n 5 15, 32 and then
constructing a line L through A14, 222 perpendicular to this normal.
6
4
2
O(0, 0)
–2 0
–2
y
normal axis
P (x, y)
n = (5, 3)
x
2
4
6
A(4, –2)
–4
–6
L
Method 1:
!
Let P1x, y2 be any point on the required line L other than A, and AP is a vector
parallel to L.
!
AP 5 1x 2 4, y 2 122 2 2 5 1x 2 4, y 1 2 2.
!
!
!
!
Since n and AP are perpendicular, n # AP 5 0.
Therefore, 15, 32 # 1x 2 4, y 1 2 2 5 0 or 51x 2 4 2 1 31y 1 2 2 5 0.
Thus, 5x 2 20 1 3y 1 6 5 0 or 5x 1 3y 2 14 5 0.
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Method 2:
!
Since n 5 15, 32, the Cartesian equation of the line is of the form
5x 1 3y 1 C 5 0, with C to be determined. Since the point A14, 22 2
is a point on this line, it must satisfy its equation,
i.e., 5142 1 31222 1 C 5 0
So C 5 214, and 5x 1 3y 2 14 5 0.
Using either method, the required Cartesian equation is 5x 1 3y 2 14 5 0.
Since it has been established that the line with equation Ax 1 By 1 C 5 0 has a
!
normal vector of n 5 1A, B2, this now provides an easy test to determine whether
lines are parallel or perpendicular. The tests for whether lines are parallel or
perpendicular are shown in the following box.
Parallel and Perpendicular Lines and their Normals
!
!
If the lines L1 and L2 have normals n1 and n2 , respectively, we know the
following.
1. The two lines are parallel if and only if their normals are scalar multiples
!
!
i.e. n1 5 kn2 , k [ R, k Þ 0.
2. The two lines are perpendicular if and only if their dot product is zero
! !
i.e. n1 # n2 5 0.
Two perpendicular lines
y
L1
L2
Two parallel lines
y L
1
L2
n1
n1
x
x
n2
n2
normal axis
n1 = kn2
normal axis L 1
normal axis L 2
n1 . n2 = 0
The following example demonstrates these ideas.
EXAMPLE 5
Reasoning about parallel and perpendicular lines in R 2
a. Show that the lines L1 :3x 2 4y 2 6 5 0 and L2 :6x 2 8y 1 12 5 0 are
parallel and non-coincident.
b. For what value of k are the lines L3 : kx 1 4y 2 4 5 0 and
L4 :3x 2 2y 2 3 5 0 perpendicular lines?
18
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Solution
!
a. The lines are parallel because, when the two normals, n1 5 13, 242
!
and n2 5 16, 282 are compared, the two vectors are scalar multiples
!
!
n2 5 16, 28 2 5 213, 242 5 2n1 . The lines are non-coincident, since there is
no value of t such that 6x 2 8y 1 12 5 t13x 2 4y 2 62 . In simple terms, lines
can only be coincident if their equations are scalar multiples of each other.
!
b. If the lines are perpendicular, then the normal vectors n3 5 1k, 4 2 and
!
n4 5 13, 22 2 have a dot product equal to zero, i.e., 1k, 4 2 # 13, 22 2 5 0 or
3k 2 8 5 0, k 5 83. This implies that the lines 3x 2 2y 2 3 5 0 and
8
x 1 4y 2 4 5 0 are perpendicular.
3
The following investigation helps in understanding the relationship between
normals and perpendicular lines.
INVESTIGATION
A. A family of lines has kx 2 2y 2 4 5 0 as its equation. On graph paper,
sketch the three members of this family when k 5 1, k 5 21, and k 5 2.
B. What point do each of the three lines you sketched in part A have in
common?
C. A second family of lines has 4x 2 ty 2 8 5 0 as its equation. Sketch the
three members of this family on the same grid for t 5 22, t 5 2, and
t 5 24.
D. What points do the lines in part C have in common?
E. Select the three pairs of perpendicular lines from the two families and verify
that you are correct by calculating the dot products of their respective normals.
F. By selecting different values for k and t, determine another pair of lines that
are perpendicular.
G. In general, if you are given a line in R2, how many different lines is it possible
to draw through a particular point perpendicular to the given line? Explain
your answer.
IN SUMMARY
Key Idea
• The Cartesian (or scalar) equation of a line in R2 is Ax 1 By 1 C 5 0 where
!
n 5 1A, B2 is a normal to the line.
(continued)
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Need to Know
!
!
• Two lines are parallel if and only if n1 5 kn2 .
! !
• Two lines are perpendicular if and only if n1 # n2 5 0.
• The angle between two lines is defined by the angle
between the two lines
!# !
!
a
b
!
direction vectors a and b where u 5 cos21 a ! ! b .
@a @ @b @
Exercise 8.2
PART A
5
1. A line has y 5 26 x 1 9 as its Cartesian equation.
a.
b.
c.
d.
Give a direction vector for a line that is parallel to this line.
Give a direction vector for a line that is perpendicular to this line.
Give the coordinates of a point on the given line.
In both vector and parametric form, give the equations of the line parallel
to the given line passing through A17, 92.
e. In both vector and parametric form, give the equations of the line
perpendicular to the given line that passes through B122, 1 2 .
2. a. Draw a sketch of the line defined by the equation
!
r 5 12, 1 2 1 s122, 52 , s [ R.
!
b. On the same axes, draw a sketch of the line q 5 122, 52 1 t12, 12 , t [ R.
3. For each of the given lines, determine the vector and parametric equations.
7
a. y 5 x 2 6
8
C
3
b. y 5 x 1 5
2
c. y 5 21
d. x 5 4
4. Explain how you can show that the lines with equations x 2 3y 1 4 5 0 and
6x 2 18y 1 24 5 0 are coincident.
5. Two lines have equations 2x 2 3y 1 6 5 0 and 4x 2 6y 1 k 5 0.
a. Explain with the use of normal vectors why these lines are parallel.
b. For what value of k will these lines be coincident?
PART B
6. Determine the Cartesian equation for the line with a normal vector of 14, 52
that passes through the point A121, 52.
7. A line passes through the points A123, 5 2 and B122, 42. Determine the
Cartesian equation of the line containing these two points.
8. Determine the equation of the line in Cartesian form that is perpendicular to
the line 2x 2 4y 1 7 5 0 and that passes through the point P17, 2 2 .
20
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9. A line has parametric equations x 5 3 2 t, y 5 22 2 4t, t [ R.
a. Draw a sketch of this line.
b. Determine a Cartesian equation for this line.
10. For each of the following pairs of lines, determine the size of the acute angle
to the nearest degree that is created by their intersection.
a. 1x, y2 5 13, 6 2 1 t12, 252 and 1x, y2 5 123, 42 2 t124, 212
b. x 5 2 2 5t, y 5 3 1 4t and x 5 21 1 t, y 5 2 2 6t
c. y 5 0.5x 1 6 and y 5 20.75x 2 1
d. 1x, y2 5 121, 212 1 t12, 42 and 2x 2 4y 5 8
e. x 5 2t, y 5 1 2 5t and 1x, y2 5 14, 0 2 1 t124, 1 2
f. x 5 3 and 5x 2 10y 1 20 5 0
11. The angle between any pair of lines in Cartesian form is also the angle
between their normal vectors. For the lines x 2 3y 1 6 5 0 and
x 1 2y 2 7 5 0, do the following.
a. Draw a sketch of the lines.
b. Determine the acute and obtuse angles between these two lines to the
nearest tenth of a degree.
T
12. The line segment joining A123, 22 and B18, 4 2 is the hypotenuse of a right
triangle. The third vertex, C, lies on the line with this vector equation:
1x, y2 5 126, 6 2 1 t123, 24 2
a. Determine the coordinates of C.
b. Illustrate with a diagram.
c. Use vectors to show that /ABC 5 90°.
PART C
!
!
13. Lines L1 and L2 have n1 and n2 as their respective normals. Prove that the
angle between the two lines is the same as the angle between the two normals.
y
u
B
n1
C
n2
A
L2
O
x
L1
(Hint: Show that /AOC 5 180° 2 u by using the fact that the sum of the
angles in a quadrilateral is 360°.)
14. The lines x 2 y 1 1 5 0 and x 1 ky 2 3 5 0 have an angle of 60° between
them. For what value of k is this true?
NEL
Section 8.3 Vector, Parametric, and Symmetric
CHAPTER 8
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Section 8.3—Vector, Parametric, and
Symmetric Equations of
a Line in R3
In Section 8.1, vector and parametric equations for a line in R2 were discussed. In
this section, this discussion is continued, but instead of R2, we now examine lines
in R3.
The derivation and form of the vector equation for a line in R3 is the same as in
R2. If we wish to find a vector equation for a line in R3, it is necessary that either
two points or a point and a direction vector be given. If we are given two points
and wish to determine a direction vector for the corresponding line, the
coordinates of this vector must first be calculated.
EXAMPLE 1
Determining a direction vector of a line in R3
A line passes through the points A121, 3, 5 2 and B123, 3, 242. Calculate
possible direction vectors for this line.
Solution
!
A possible direction vector is m 5 121 2 1232 , 3 2 3, 5 2 12422 5 12, 0, 9 2.
In general, any vector of the form t12, 0, 9 2, t [ R, t Þ 0 can be used as a
direction vector for this line. As before, the best choice for a direction vector is
one in which the direction numbers are integers with common divisors removed.
This implies that either 12, 0, 92 or 122, 0, 29 2 are the best choices for a
!
direction vector for this line. Generally speaking, if a line has m 5 1a, b, c2
as its direction vector, then any scalar multiple of this vector of the form
t1a, b, c2, t [ R, t Þ 0 can be used as a direction vector.
Vector and Parametric Equations of Lines in R3
Consider the following diagram.
z
P(x, y, z)
P0(x0, y0, z0)
r
r0
m
O
y
x
22
In determining the vector equation of the line passing through P0 and P, the
!
point P0 1x0, y0, z0 2 is a given point on the line, and m 5 1a, b, c2 is its
8.3
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direction
vector. If P1x, y, z2 represents a general point on the line, then
!
P0P 5 1x 2 x0, y 2 y0, z 2 z0 2 is a direction vector for this line. This allows us
to form the vector equation of the line.
!
!
!
In triangle OP0P, OP 5 OP0 1 P0P .
!
!
!
!
!
!
!
Since OP0 5 r0 and P0P 5 tm , the vector equation of the line is r 5 r0 1 tm , t [ R.
In component form, this can be written as 1x, y, z2 5 1x 0, y0, z 0 2 1 t1a, b, c2, t [ R.
The parametric equations for the line are found by equating the respective x, y, and z
components giving x 5 x 0 1 ta, y 5 y0 1 tb, z 5 z 0 1 tc, t [ R.
Vector and Parametric Equations of a Line in R3
!
!
!
Vector Equation: r 5 r0 1 tm , t [ R
Parametric Equations: x 5 x0 1 ta, y 5 y0 1 tb, z 5 z0 1 tc, t [ R
!
where r0 5 1x0, y0, z0 2, the vector from the origin to a point on the line and
!
m 5 1a, b, c2 is a direction vector of the line.
EXAMPLE 2
Representing the equation of a line in R3 in vector and parametric form
Determine the vector and parametric equations of the line passing through
P122, 3, 52 and Q122, 4, 21 2.
Solution
!
A direction vector is m 5 122 2 122 2, 3 2 4, 5 2 1212 2 5 10, 21, 6 2. A vector
!
equation is r 5 122, 3, 5 2 1 s10, 21, 62, s [ R, and its parametric equations are
x 5 22, y 5 3 2 s, z 5 5 1 6s, s [ R. It would also have been correct to choose
any multiple of 10, 21, 6 2 as a direction vector and any point on the line. For
!
example, the vector equation, r 5 122, 4, 21 2 1 t10, 22, 122 , t [ R would also
have been correct.
Since a vector equation for a line can be written in many ways, it is useful to be
able to tell if different forms are actually equivalent. In the following example, an
algebraic approach to this problem is considered.
EXAMPLE 3
Reasoning to establish the equivalence of two lines
a. Show that the following are vector equations for the same line:
!
L 1 : r 5 121, 0, 4 2 1 s121, 2, 5 2 , s [ R and
!
L 2 : r 5 14, 210, 2212 1 m122, 4, 10 2, m [ R.
b. Show that the following are vector equations for different lines,
!
L 3 : r 5 11, 6, 12 1 l121, 1, 22, l [ R and
1 1
!
L 4 : r 5 123, 10, 122 1 k a , 2 , 21 b , k [ R.
2 2
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Solution
a. Since the direction vectors are parallel, i.e., 2121, 2, 52 5 122, 4, 10 2, this
means that the two lines are parallel. To show that the equations are equivalent,
we must show that a point on one of the lines is on the second line. This is
based on the logic that, if the lines are parallel and they share a common point,
then the two equations must represent the same line. We’ll check whether
14, 210, 221 2 is on L1 by substituting into its vector equation.
Therefore, 14, 210, 221 2 5 121, 0, 42 1 s121, 2, 52 .
Using the x component, we find 4 5 21 2 s, or s 5 25. Substituting s 5 25
into the above equation, 121, 0, 42 1 2 5121, 2, 5 2 5 14, 210, 221 2. This
shows that the point 14, 210, 2212 is on L1.
Since the lines have the same direction, and a point on one line is on the
second, the two given equations represent the same line.
b. The first check is to compare the direction vectors of the two lines. Since
22Q2, 22, 21R 5 121, 1, 2 2, the lines must be parallel. As in part a.,
1
1
123, 10, 122 must be a point on L3 in order for the equations to be equivalent.
Therefore, 123, 10, 12 2 5 11, 6, 1 2 1 l121, 1, 22 must give a consistent value
of l for each component. If we solve, this gives an inconsistent result since
11
l 5 4 for the x and y components, and 12 5 1 1 2l, or l 5 2 for the z component.
This demonstrates that the two equations are not equations for the same line.
Symmetric Equations of Lines in R3
We introduce a new form for a line in R3, called its symmetric equation. The
symmetric equation for a line is derived from using its parametric equations and
solving for the parameter in each component. This is shown in the following.
x 2 x0
x 5 x0 1 ta 4 t 5
,a Þ 0
a
y 2 y0
y 5 y0 1 tb 4
5 t, b Þ 0
b
z 2 z0
z 5 z0 1 tc 4
5 t, c Þ 0
c
x2x
y2y
z2z
Combining these statements gives a 0 5 b 0 5 c 0, a, b, c Þ 0.
These equations are called the symmetric equations of a line in R3.
Symmetric Equations of a Line in R3
x 2 x0
y 2 y0
z 2 z0
5
, a Þ 0, b Þ 0, c Þ 0.
5
a
c
b
1x0, y0, z0 2 is the vector from the origin to a point on the line and 1a, b, c2 is a
direction vector of the line.
24
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Representing the equation of a line in R3 in symmetric form
a. Write the symmetric equations of the line passing through the points
A121, 5, 72 and B13, 24, 82 .
b. Write the symmetric equations of the line passing through P122, 3, 12 and
Q14, 3, 252.
c. Write the symmetric equations of the line passing through the points
X121, 2, 52 and Y121, 3, 9 2.
Solution
a. A direction vector for this line is
!
m 5 121 2 3, 5 2 124 2, 7 2 8 2 5 124, 9, 21 2 . Using the point A121, 5, 7 2,
the parametric equations of the line are x 5 21 2 4t, y 5 5 1 9t and
z 5 7 2 t, t [ R. Solving each equation for t gives the required symmetric
x11
y25
z27
equations, 24 5 9 5 21 . It is usually not necessary to first find the
parametric equations before finding symmetric equations. The symmetric equations
of a line can be written by inspection if the direction vector and a point on the
line are known. The symmetric equations of this line by inspection are
x23
z14
z28
5 9 5 21 .
24
!
b. A direction vector for the line is m 5 122 2 4, 3 2 3, 1 2 12522 5 126, 0, 6 2 .
The vector 11, 0, 21 2 will be used as the direction vector. In a situation
like this, where the y direction number is 0, the equation is written as
x12
z21
5 21 , y 5 3.
1
!
c. A direction vector for the line is m 5 121 2 1212 , 2 2 3, 5 2 9 2 5 10, 21, 24 2.
y22
z25
Possible symmetric equations are 21 5 24 , x 5 21.
IN SUMMARY
!
!
• In R3 if r0 5 1x0, y0, z0 2 is determined by a point on a line and m 5 1a, b, c2
is a direction vector of the same line then
!
!
!
3
• The vector equation of a line in R is r 5 r 0 1 tm , t [ R.
• The parametric form of the equation of a line is x 5 x0 1 ta, y 5 y0 1 tb,
and z 5 z0 1 tc, t [ R.
•
The symmetric form of the equation of a line is
x 2 x0
y2y
z2z
5 b 0 5 c 0 15t2, a Þ 0, b Þ 0, c Þ 0.
a
Need to Know
• Knowing one of these forms of the lines equation enables you to find the other
two, since all three forms depend on the same information about the line.
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Exercise 8.3
PART A
1. State the coordinates of a point on each of the given lines.
!
a. r 5 123, 1, 8 2 1 s121, 1, 9 2, s [ R
y11
x21
z23
5
5
2
1
21
c. x 5 22 1 3t, y 5 1 1 24t, z 5 3 2 t, t [ R
x12
z21
d.
5
, y 5 23
21
2
e. x 5 3, y 5 22, z 5 21 1 2k, k [ R
b.
f.
x 2 13
1
2
5
y 1 34
21
4
5
z 2 25
1
2
2. State a direction vector for each line in question 1, making certain that the
components for each are integers.
PART B
3. A line passes through the points A121, 2, 4 2 and B13, 23, 52.
a. Write two vector equations for this line.
b. Write the two sets of parametric equations associated with the vector
equations found in 3. a.
4. A line passes through the points A121, 5, 242 and B12, 5, 24 2 .
a. Write a vector equation for the line containing these points.
b. Write parametric equations corresponding to the vector equation you
found in 4. a.
c. Explain why there are no symmetric equations for this line.
K
5. State vector, parametric, and symmetric equations for each of the following
lines.
a. the line passing through the point P121, 2, 1 2 with a direction vector of
13, 22, 12
b. the line passing through the points A121, 1, 0 2 and B121, 2, 12
c. the line passing through B122, 3, 02 and parallel to the line passing
through the points M122, 22, 1 2 and N122, 4, 7 2
d. the line passing through the points D121, 0, 0 2 and E121, 1, 02
e. the line passing through the points X124, 3, 0 2 and O10, 0, 02
f. the line passing through Q11, 2, 4 2 and parallel to the z-axis.
26
8.3
V E C TO R , PA R A M E T R I C , A N D S Y M M E T R I C E Q UAT I O N S O F A L I N E I N R3
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6. a. Find parametric equations for each of the following lines:
x16
10
7
7
11
and x 1
5y2
5z2
5y2
, z 5 5.
1
21
1
1
21
b. Determine the angle between the two lines.
C
7. Show that the following two sets of symmetric equations represent the same
straight line:
x17
y11
z25
x21
y21
z23
5 2 5 22 and 24 5 21 5 1 .
8
8. a. Show that the points A16, 22, 15 2 and B1215, 5, 227 2 lie on the line that
passes through (0, 0, 3) and which has the direction vector 123, 1, 26 2.
b. Use parametric equations with suitable restrictions on the parameter to
describe the line segment from A to B.
A
9. Determine the value of k for which the lines
x13
z
5 1, y 5 21 are perpendicular.
22
x21
y22
z11
5 2 5 k 2 1 and
k
10. Determine the coordinates of three different points on each line.
a. 1x, y, z2 5 14, 22, 5 2 1 t124, 26, 8 2
b. x 5 24 1 5s, y 5 2 2 s, z 5 9 2 6s
y22
x11
z
5
5
3
21
4
y22
z23
d. x 5 24,
5
3
5
c.
11. For each of the equations in question 10, express it in two other equivalent
forms.
PART C
12. Find the parametric equations of the line perpendicular to the two lines
x
y 1 10
z12
x25
y25
z15
5 27 5 3 and 3 5 2 5 4 and which also passes
24
through the point 12, 25, 02 .
T
13. A line with parametric equations x 5 10 1 2s, y 5 5 1 s, z 5 2, s [ R,
intersects a sphere with the equation x 2 1 y 2 1 z 2 5 9 at the points A and B.
Determine the coordinates of these points.
14. You are given the two lines L 1 : x 5 4 1 2t, y 5 4 1 t, z 5 23 2 t, t [ R,
and L 2 : x 5 22 1 3s, y 5 27 1 2s, z 5 2 2 3s, s [ R. If the point P! 1 lies
on L1, and P2 is on L2, find the coordinates of these two points! if P1P2 is
perpendicular to each of the two lines. (Hint: The vector P1P2 is
perpendicular to the direction vector of each of the two lines.)
15. Determine the angle formed by the intersection of the lines defined by
x21
3
2
1
5y1
, z 5 23 and x 2
5y1
5 1z .
2
1
3
2
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Mid-Chapter Review
1. Name three points on each of the following lines.
a. x ⫽ 2t ⫺ 5, y ⫽ 3t ⫹ 1, t [ R
c. 3x ⫹ 5y ⫺ 8 ⫽ 0
!
b. r ⫽ 12, 3 2 ⫹ s13, ⫺22 , s [ R
d.
y⫹2
x⫺1
z⫺5
⫽
⫽
3
2
1
2. Find x- and y-intercepts for each of the following lines.
!
a. r ⫽ 13, 1 2 ⫹ t1⫺3, 5 2 , t [ R
b. x ⫽ ⫺6 ⫹ 2s and y ⫽ 3 ⫺ 2s, s [ R
!
!
3. Two lines L 1 : r ⫽ 15, 3 2 ⫹ p1⫺4, 7 2 , p [ R, and L 2 : r ⫽ 15, 3 2 ⫹ q12, 12 ,
q [ R, intersect at the point with coordinates 15, 3 2. What is the angle
between L 1 and L 2?
!
4. Determine the angle that the line with equation r ⫽ t14, ⫺5 2, t [ R, makes
with the x- and y-axis, respectively.
5. Determine a Cartesian equation for the line passing through the point
with coordinates 14, ⫺3 2 and which is perpendicular to the line
!
r ⫽ 12, ⫺3 2 ⫹ t15, ⫺72, t [ R.
6. Determine an equation in symmetric form for a line parallel to
x⫺3
⫺5
7
⫽ y ⫺4
⫽z⫹
and passing through 10, 0, 2 2 .
3
4
7. Determine parametric equations for the line passing through 11, 2, 5 2 and
parallel to the line passing through K12, 4, 52 and L13, ⫺5, 6 2.
8. Determine direction angles for the line with equations x ⫽ 5 ⫹ 2t,
y ⫽ 12 ⫺ 8t, z ⫽ 5 ⫹ 7t, t [ R.
9. Determine an equation in symmetric form for the line passing through
P 13, ⫺4, 62 and having direction angles 60°, 90°, and 30°.
10. Write an equation in parametric form for each of the three coordinate axes
in R3.
!
11. The two lines with equations r ⫽ 11, 2, ⫺4 2 ⫹ t1k ⫹ 1, 3k ⫹ 1, k ⫺ 3 2,
t [ R, and x ⫽ 2 ⫺ 3s, y ⫽ 1 ⫺ 10s, z ⫽ 3 ⫺ 5s, s [ R, are given.
a. Determine a value for k if these lines are parallel.
b. Determine a value for k if these lines are perpendicular.
12. Determine the perimeter and area of the triangle whose vertices are the origin
and the x- and y-intercepts of the line
28
MID-CHAPTER REVIEW
x⫺6
⫹8
⫽ y ⫺2
.
3
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13. The Cartesian equation of a line is given by 3x ⫹ 4y ⫺ 24 ⫽ 0.
a. Determine a vector equation for this line.
b. Determine the parametric equations of this line.
c. Determine the acute angle that this line makes with the x-axis.
d. Determine a vector equation of the line that is perpendicular to the given
line that passes through the origin.
14. Determine the scalar, vector and parametric equations of the line that passes
through A1⫺4, 6 2 and B18, 4 2.
15. Determine a unit vector normal to the line defined by the parametric equations
x ⫽ 1 ⫹ 2t and y ⫽ ⫺5 ⫺ 4t.
16. Determine the parametric equations of the line
a. that pass through 1⫺5, 10 2 with a slope of ⫺3
2
b. that passes through 11, ⫺1 2 and is perpendicular to the line
1x, y2 ⫽ 14, ⫺6 2 ⫹ t12, ⫺2 2
c. through the 10, 7 2 and 10, 102
17. Given the line 1x, y, z2 ⫽ 112, ⫺8, ⫺42 ⫹ t1⫺3, 4, 22 ,
a. determine the intersections with the coordinate planes, if any
b. determine the intercepts with the coordinate axes, if any
c. graph the line in an x-, y-, z-coordinate system.
18. For each of the following, determine vector, parametric, and if possible,
symmetric
! equations of the line that passes through P0 and has direction
vector d .
!
a. P0 ⫽ 11, ⫺2, 8 2 , d ⫽ 1⫺5, ⫺2, 1 2
!
d ⫽ 12, 4, 6 2
b. P0 ⫽ 13, 6, 9 2,
!
d ⫽ 1⫺1, 5, 1 2
c. P0 ⫽ 10, 0, 6 2,
!
d ⫽ 10, 0, ⫺2 2
d. P0 ⫽ 12, 0, 0 2,
19. Determine a vector equation of the line that passes through the origin and is
parallel to the line through the points 1⫺4, 5, 6 2 and 16, ⫺5, 4 2.
20. Determine the parametric equations of the line that is the perpendicular to
and passes through the midpoint of the line passing through 12, 6, 10 2 and
1⫺4, 4, ⫺8 2.
21. The symmetric equations of two lines are given. Show that these lines are parallel.
2
3
⫺4
x⫹1
y⫺2
z⫹1
L1 : x ⫺
⫽y⫹
⫽ z ⫺5
and L2 : ⫺3 ⫽ ⫺9 ⫽ 15 .
1
3
22. Does the point D17, ⫺1, 8 2 lie on the line with symmetric equations
x⫺4
2
6
⫽y⫹
⫽z⫺
? Explain.
3
1
2
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Section 8.4—Vector and Parametric
Equations of Planes
In the previous section, the vector, parametric, and symmetric equations for lines
in R3 were developed. In this section, we will develop vector and parametric
equations for planes in R3. Planes are flat surfaces that extend infinitely far in all
directions. In order to represent planes, parallelograms are used to represent a
small part of the plane and are designated with the Greek letter p. This is the usual
method for representing planes. In real life, part of a plane might be represented
by the top of a desk, by a wall, or by the ice surface of a hockey rink.
Before developing the equation of a plane, we start by showing that planes can be
determined in essentially four ways.
A plane is determined if we are given any of the following:
a. a line and a point not on the line
b. three noncollinear points (three points not on a line)
c. two intersecting lines
d. two parallel, and non-coincident, lines
a.
L
p
b.
A
P0
L1 p
L2
Two intersecting lines
p
C
Three non collinear points
A line and a point
not on the line
c.
B
d.
L2
L1
p
Two parallel, and
non-coincident lines
This means that, if we are given any one of these conditions, we are guaranteed
that we can form a plane, and the plane formed will be unique. For example, in
a., we are given a line L and a point P0 not on this line; there is just one plane
that can be formed using this point and this line.
Linear Combinations and Their Relationship to Planes
The ideas of linear combination and spanning sets are the two concepts needed to
understand how to obtain the vector and parametric
! equations for planes. For
!
example, suppose vectors a ⫽ 11, 2, ⫺1 2 and b ⫽ 10, 2, 1 2 and a linear
!
combination of these vectors, i.e., P0 P ⫽ s11, 2, ⫺1 2 ⫹ t10, 2, 1 2, s, t [ R, are
formed. As different values are chosen for s and t, a new vector is formed. Different
values for these parameters have been selected, and !the corresponding calculations
have been done in the table shown, with vector P0 P also calculated.
30
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V E C TO R A N D PA R A M E T R I C E Q UAT I O N S O F P L A N E S
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t
s(1, 2, ⴚ1) ⴙ t(0, 2, 1), s, t [ R
⫺2
1
⫺211, 2, ⫺12 ⫹ 110, 2, 12
4
⫺3
411, 2, ⫺12 ⫺ 310, 2, 12
10
⫺7
1011, 2, ⫺12 ⫺ 710, 2, 12
⫺2
⫺1
⫺211, 2, ⫺12 ⫺ 110, 2, 12
s
!
P0P
1⫺2, ⫺4, 22 ⫹ 10, 2, 12 ⫽ 1⫺2, ⫺2, 32
14, 8, ⫺42 ⫹ 10, ⫺6, ⫺32 ⫽ 14, 2,⫺72
110, 20, ⫺102 ⫹ 10, ⫺14, ⫺72 ⫽ 110, 6, ⫺172
1⫺2, ⫺4, 22 ⫹ 10, ⫺2, ⫺12 ⫽ 1⫺2, ⫺6, 12
!
!
!
P0 P is on the plane determined by the vectors a and b , as is its head. The
parameters s and t are chosen from the set of real numbers, meaning that there
are an infinite number of vectors formed by selecting all possible combinations
of s and t. Each one of these vectors is different, and every point on the plane
can be obtained by choosing appropriate parameters. This observation is used in
developing the vector and parametric equations of a plane.
!
!
In the following, two noncollinear vectors,
and
are given. The linear
a
b
!
!
combinations of these vectors,
form
the
diagonals
of the parallelograms
⫹
tb
,
sa
!
!
determined by sa and tb .
P
tb
sa + tb
b
P0
EXAMPLE 1
sa
a
Developing the Vector and Parametric Equations of a Plane
!
!
Two noncollinear vectors 1a and b 2 are given, with their tails at point P0.
Determine the vector and parametric equations of the plane p formed by taking
all linear combinations of these vectors.
Solution
We start by drawing !a parallelogram to represent part of the plane p. This plane
!
contains P0, a , and b .
z
b
P p
P0
r0
O
r
a
y
x
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!
From the diagram, it can be seen that vector r0 represents the vector for a
!
particular point P!0 on the plane, and r represents the vector for any point
! P
!
on the
plane.
is
on
the
plane
and
is
a
linear
combination
of
and
P
a
. i.e.,
P
b
0!
!
!
the triangle! law of addition in triangle OP0P,
P0 P! ⫽ sa ⫹
! tb , s, ! t [ R. Using
!
!
!
OP ⫽ OP0 ⫹ P0P . Thus, r ⫽ r0 ⫹ sa ⫹ tb , s, t [ R.
!
!
!
!
The vector equation for the plane is r ⫽ r0 ⫹ sa ⫹ tb , s, t [ R, and can be used
to generate parametric equations for the plane.
!
!
!
!
If r ⫽ 1x, y, z 2, r0 ⫽ 1x0, y0, z0 2, a ⫽ 1a1, a2, a3 2 and b ⫽ 1b1, b2, b3 2 ,
these expressions can be substituted into the vector equation to obtain
1x, y, z2 ⫽ 1x0, y0, z0 2 ⫹ s1a1, a2, a3 2 ⫹ t1b1, b2, b3 2, s, t [ R.
Expanding, 1x, y, z2 ⫽ 1x0, y0, z0 2 ⫹ 1sa1, sa2, sa3 2 ⫹ 1tb1, tb2, tb3 2.
Simplifying, 1x, y, z2 ⫽ 1x0 ⫹ sa1 ⫹ tb1, y0 ⫹ sa2 ⫹ tb2, z0 ⫹ sa3 ⫹ tb3 2 .
Equating the respective components gives the parametric equations
x ⫽ x0 ⫹ sa1 ⫹ tb1, y ⫽ y0 ⫹ sa2 ⫹ tb2, z ⫽ z0 ⫹ sa3 ⫹ tb3, s, t [ R.
Vector and Parametric Equations for a Plane in R3
!
In R3,! r0 ⫽ 1x0, y0, z0 2 is determined by a point on a plane and a ⫽ 1a1, a2, a3 2
and b ⫽ 1b1, b2, b3 2 are vectors that lie on the! same plane as the point.
!
!
!
Vector Equation of a Plane: r ⫽ r0 ⫹ sa ⫹ tb , s, t [ R
Parametric Equations for a Plane: x ⫽ x 0 ⫹ sa 1 ⫹ tb 1,
y ⫽ y0 ⫹ sa 2 ⫹ tb 2,
z ⫽ z 0 ⫹ sa 3 ⫹ tb 3, s, t [ R.
!
!
The vectors a and b are the direction vectors for the plane. In determining the
equation of a plane, it is necessary to have two direction vectors. As will be seen
in the examples, any pair of noncollinear and coplanar vectors can be used as
direction vectors of a plane. The vector equation for a plane always requires two
parameters, s and t, each of which are real numbers. Because two parameters are
required to define a plane, the plane is described as two-dimensional. Earlier we
!
!
!
saw that the vector equation of a line, r ⫽ r0 ⫹ tm , t [ R, required just one
parameter. For this reason, a line is described as one-dimensional. A second
observation about the vector equation of the plane is that there is a one-to-one
correspondence between the chosen parametric values (s, t) and points on the
plane. Each time values for s and t are selected, this generates a different point
on the plane, and because s and t can be any real number, this will generate all
points on the plane.
32
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V E C TO R A N D PA R A M E T R I C E Q UAT I O N S O F P L A N E S
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After deriving vector and parametric equations of lines, a symmetric form was
also developed. Although it is possible to derive vector and parametric equations
for planes, it is not possible to derive a corresponding symmetric form for a plane.
The following example shows how to derive an equation of a plane passing
through three points.
EXAMPLE 2
Selecting a strategy to represent the vector and parametric
equations of a plane
a. Determine a vector equation and the corresponding parametric equations for
the plane that contains the points A1⫺1, 3, 8 2 , B1⫺1, 1, 0 2, and C14, 1, 1 2 .
b. Do either of the points P114, 1, 32 or Q114, 1, 5 2 lie on this plane?
Solution
a. In finding the required vector equation, it is necessary to have two direction
vectors for the plane. The following shows the calculations for each of the
direction vectors.
Direction Vector 1
In calculating the first direction vector, any two points can be used and a
position vector
! determined. If the points A1⫺1, 3, 8 2 and B1⫺1, 1, 0 2 are
used, then AB ⫽ 1⫺1 ⫺ 1⫺1 2, 1 ⫺ 3, 0 ⫺ 82 ⫽ 10, ⫺2, ⫺82.
Since, 10, ⫺2, ⫺82 ⫽ ⫺210, 1, 4 2 , a possible first direction vector is
!
a ⫽ 10, 1, 42.
Direction Vector 2
In finding the second direction vector,
! any two points can be chosen other than A
and B. If B and C are used, then BC ⫽ 14 ⫺ 1⫺1 2, 1 ⫺ 1, 1 ⫺ 0 2 ⫽ 15, 0, 1 2 .
!
A second direction vector is b ⫽ 15, 0, 1 2 .
In determining the actual equation for the plane, any of the points A, B, or C can
be used, and an equation for the plane is
!
r ⫽ 1⫺1, 3, 8 2 ⫹ s10, 1, 4 2 ⫹ t15, 0, 1 2, s, t [ R.
Writing the vector equation in component form will give the parametric
equations. Thus, 1x, y, z 2 ⫽ 1⫺1, 3, 8 2 ⫹ 10, s, 4s2 ⫹ 15t, 0, t2 .
The parametric equations are x ⫽ ⫺1 ⫹ 5t, y ⫽ 3 ⫹ s, and z ⫽ 8 ⫹ 4s ⫹ t,
s, t [ R.
b. If the point P114, 1, 3 2 lies on the plane, there must be parameters that
correspond to this point. To find these parameters, x ⫽ 14 and y ⫽ 1 are
substituted into the corresponding parametric equations.
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Thus, 14 ⫽ ⫺1 ⫹ 5t and 1 ⫽ 3 ⫹ s.
Solving for s and t, we find that 14 ⫹ 1 ⫽ 5t, or t ⫽ 3 and 1 ⫺ 3 ⫽ s, or
or s ⫽ ⫺2. Using these values, consistency will be checked with the z
component. If s ⫽ ⫺2 and t ⫽ 3 is substituted into z ⫽ 8 ⫹ 4s ⫹ t, then
z ⫽ 8 ⫹ 41⫺2 2 ⫹ 3 ⫽ 3. Since the z value for the point is also 3, this tells us
that the point with coordinates P114, 1, 32 is on the given plane.
From this, it can be seen that the parametric values used for the x and y
components, i.e., s ⫽ ⫺2 and t ⫽ 3, will not produce consistent values for
z ⫽ 5. So the point P114, 1, 5 2 is not on the plane.
In the following example, we will show how to use vector and parametric
equations to find the point of intersection of planes with the coordinate axes.
EXAMPLE 3
Solving a problem involving a plane
!
A plane p has r ⫽ 16, ⫺2, ⫺3 2 ⫹ s11, 3, 02 ⫹ t12, 2, ⫺1 2, s, t [ R, as its
equation. Determine the point of intersection between p and the z-axis.
Solution
We start by writing this equation in parametric form. The parametric equations of
this plane are x ⫽ 6 ⫹ s ⫹ 2t, y ⫽ ⫺2 ⫹ 3s ⫹ 2t, and z ⫽ ⫺3 ⫺ t.
The plane intersects the z-axis at a point having coordinates of the form
P10, 0, c2, i.e., where x ⫽ y ⫽ 0. Substituting these values into the parametric
equations for x and y gives, 0 ⫽ 6 ⫹ s ⫹ 2t and 0 ⫽ ⫺2 ⫹ 3s ⫹ 2t. Simplifying
gives the following system of equations
1
s ⫹ 2t ⫽ ⫺6
2
3s ⫹ 2t ⫽ 2
Subtracting equation
1
from equation
2
gives 2s ⫽ 8, so s ⫽ 4.
The value of t is found by substituting into either of the two equations. Using
equation 1 , 4 ⫹ 2t ⫽ ⫺6, or t ⫽ ⫺5.
Solving for z using the equation for the third component, we find that
z ⫽ ⫺3 ⫺ 1⫺52 ⫽ 2.
Thus the plane cuts the z-axis at the point P10, 0, 2 2.
34
8.4
V E C TO R A N D PA R A M E T R I C E Q UAT I O N S O F P L A N E S
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Representing the equations of a plane from a point and a line
Determine the vector and parametric equations for the plane containing the point
!
P11, ⫺5, 9 2 and the line L : r ⫽ 11, 1, 1 2 ⫹ s1⫺1, 1, 0 2 , s [ R.
Solution
In the following diagram, a representation of the line L and the point P are given.
L: r = (1, 1, 1) + s(–1, 1, 0)
P(1, –5, 9)
A(1, 1, 1)
p
In order to find the equation of this plane, it is necessary to find two direction vectors
!
and a point on the plane. The line L : r ⫽ 11, 1, 1 2 ⫹ s1⫺1, 1, 0 2 , s [ R, gives a
point and one direction vector, so all that
! is required is a second direction vector.
Using A11, 1, 1 2 and P11, ⫺5, 92, AP ⫽ 11 ⫺ 1, ⫺5 ⫺ 1, 9 ⫺ 1 2 ⫽ 10, ⫺6, 8 2 ⫽
⫺210, 3, ⫺4 2. This is a second direction vector. The equation for the plane is,
!
r ⫽ 11, 1, 1 2 ⫹ s1⫺1, 1, 02 ⫹ t10, 3, ⫺4 2 , s, t [ R.
The corresponding parametric equations are, x ⫽ 1 ⫺ s, y ⫽ 1 ⫹ s ⫹ 3t, and
z ⫽ 1 ⫺ 4 t, t [ R.
IN SUMMARY
Key Ideas
!
• In R3, if r0 ⫽ 1x0, y0, z0 2 is determined by a point on a plane and
!
!
a ⫽ 1a1, a2, a3 2 and b ⫽ 1b1, b2, b3 2 are direction vectors that lie on the
same plane as the point then
!
!
!
!
3
• The vector equation of a plane in R is r ⫽ r0 ⫹ ta ⫹ sb , t, s [ R.
• The parametric form of the equation of a plane is x ⫽ x0 ⫹ ta1 ⫹ sb1,
y ⫽ y0 ⫹ ta2 ⫹ sb2, z ⫽ z0 ⫹ ta3 ⫹ sb3, t, s [ R.
• There are no symmetric equations of a plane.
Need to Know
• Replacing the parameters in the vector and parametric equations of a plane
with numbers generates points on the plane. Because there are an infinite
number of real numbers that can be used for s and t, there are an infinite
number of points that lie on a plane.
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Exercise 8.4
PART A
1. State which of the following equations define lines and which define planes.
Explain your answer.
!
a. r ⫽ 11, 2, 3 2 ⫹ s11, 1, 02 ⫹ t13, 4, ⫺6 2, s, t [ R
!
b. r ⫽ 1⫺2, 3, 0 2 ⫹ m13, 4, 72 , m [ R
c. x ⫽ ⫺3 ⫺ t, y ⫽ 5, z ⫽ 4 ⫹ t, t [ R
!
d. r ⫽ m14, ⫺1, 22 ⫹ t14, ⫺1, 52 , m, t [ R
!
2. A plane has vector equation r ⫽ 12, 1, 3 2 ⫹ s Q13, ⫺2, 34R ⫹ t16, ⫺12, 30 2,
s, t [ R.
a. Express the first direction vector with only integers.
b. Reduce the second direction vector.
c. Write a new equation for the plane using the calculations from 2. a. and 2. b.
3. A plane has x ⫽ 2m, y ⫽ ⫺3m ⫹ 5n, z ⫽ ⫺1 ⫺ 3m ⫺ 2n, m, n [ R as its
parametric equations.
a.
b.
c.
d.
e.
C
36
8.4
By inspection, identify the coordinates of a point that is on this plane.
What are the direction vectors for this plane?
What point corresponds to the parameter values of m ⫽ ⫺1 and n ⫽ ⫺4?
What are the parametric values corresponding to the point A10, 15, ⫺7 2?
Using 3. d., explain why the point B10, 15, ⫺82 cannot be on the plane.
4. A plane passes through the points P1⫺2, 3, 12, Q1⫺2, 3, 2 2 , and R11, 0, 12 .
!
!
a. Using PQ and PR as direction vectors, write a vector equation for this plane.
!
b. Using QR and one other direction vector, write a second vector equation
for this plane.
!
5. Explain why the equation r ⫽ 1⫺1, 0, ⫺12 ⫹ s12, 3, ⫺42 ⫹ t14, 6, ⫺8 2,
s, t [ R does not represent the equation of a plane. What does this equation
represent?
PART B
6. Find vector equations and the corresponding parametric equations for each of
the following.
!
!
a. the plane with direction vectors a ⫽ 14, 1, 0 2 and b ⫽ 13, 4, ⫺1 2 passing
through the point A1⫺1, 2, 72
b. the plane passing through the points A11, 0, 02 , B10, 1, 0 2, and C10, 0, 1 2
c. the plane passing through A11, 1, 02 and B14, 5, ⫺6 2 and having direction
!
vector a ⫽ 17, 1, 2 2
V E C TO R A N D PA R A M E T R I C E Q UAT I O N S O F P L A N E S
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7. a. Find parameters corresponding to the point P15, 3, 2 2 where P is a point on
!
the plane with equation p : r ⫽ 12, 0, 1 2 ⫹ s14, 2, ⫺12 ⫹ t1⫺1, 1, 2 2,
s, t [ R.
b. Show that the point A10, 5, ⫺42 does not lie on p.
!
8. A plane has r ⫽ 1⫺3, 5, 6 2 ⫹ s1⫺1, 1, 22 ⫹ v12, 1, ⫺3 2 , s, v [ R as its
equation.
a. Give the equation of two intersecting lines that lie on this plane.
b. What point do these two lines have in common?
A
9. Determine the coordinates of the point where the plane with equation
!
r ⫽ 14, 1, 6 2 ⫹ s111, ⫺1, 3 2 ⫹ t1⫺7, 2, ⫺2 2, s, t [ R crosses the z-axis.
10. Determine the equation of the plane containing the point P1⫺1, 2, 12 and the
!
line r ⫽ 12, 1, 32 ⫹ s14, 1, 5 2, s [ R.
11. Determine the equation of the plane that contains the point A1⫺2, 2, 32 and
!
the line r ⫽ m12, ⫺1, 7 2, m [ R.
12. a. Determine two pairs of direction vectors that can be used to represent the
xy-plane in R3.
b. Write a vector and parametric equation for the xy-plane in R3.
K
13. a. Find a vector equation for the plane containing the points O10, 0, 02,
A1⫺1, 2, 52 , and C13, ⫺1, 72 .
b. Find a vector equation for the plane containing the points P1⫺2, 2, 32,
Q1⫺3, 4, 8 2 , and R11, 1, 10 2 .
c. How are the planes found in 15. a. and 15. b. related? Explain your answer.
T
14. Show that the following equations represent the same plane:
!
r ⫽ u1⫺3, 2, 4 2 ⫹ v1⫺4, 7, 1 2 , u, v [ R, and
!
r ⫽ s1⫺1, 5, ⫺3 2 ⫹ t1⫺1, ⫺5, 7 2, s, t [ R. (Hint: Express each of the
direction vectors in the first equation as a linear combination of the direction
vectors in the second equation.)
!
15. The plane with equation r ⫽ 11, 2, 3 2 ⫹ m11, 2, 5 2 ⫹ n11, ⫺1, 32 intersects
the y- and z-axes at the points A and B, respectively. Determine the equation
of the line that contains these two points.
PART C
!
!
!
16. Suppose that! the lines
and
are
defined
by
the
equations
L
⫹ sa
L
⫽
OP
r
1
2
0
!
!
!
!
and r ⫽ OP0 ⫹ tb , respectively, where s, t [ R, and a and b !are noncollinear
!
!
!
vectors. Prove that the plane defined by the equation r ⫽ OP0 ⫹ sa ⫹ tb
contains both of the given lines.
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Section 8.5—The Cartesian Equation
of a Plane
In the previous section, the vector and parametric equations of a plane were found. In
this section, we will show how to derive the Cartesian (or scalar) equation of a plane.
This process is very similar to finding the Cartesian equation of a line in R2.
p
z
normal axis
N(A, B, C)
O(0, 0, 0)
y
x
In the diagram above, a plane p is shown, along with a line L drawn from the
origin, so that L is perpendicular to the given plane. For any plane in R3, there is
only one possible line that can be drawn through the origin perpendicular to the
plane. This line is called the normal axis for the plane. The direction of the
normal axis is given by a vector joining the origin to any point on this
! axis. This
direction vector is called a normal to the plane. In the diagram, ON is a normal to
the plane because it joins the origin to N1A, B, C 2, a point on the normal axis.
Any nonzero vector is a normal to a plane if it lies along the normal axis. This
implies that an infinite number of normals exist for all planes.
A plane is completely determined when we know a point P0 1x0, y0, z0 2 on the
plane and a normal to the plane. This single idea can be used to find the Cartesian
equation of a plane.
Deriving the Cartesian Equation of a Plane
Consider the following diagram.
n = (A, B, C)
z
p
normal axis
P(x,y,z)
)
, z0
0
y
,
(x 0
P0
O
y
x
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T H E C A RT E S I A N E Q UAT I O N O F A P L A N E
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In deriving the equation of this plane, we need two points on the plane,
given) and a general point, P1x, y, z2 , different from
P0 1x0, y0, z0 2 (its coordinates
!
P0. The vector P0P ⫽ 1x ⫺ x0, y ⫺ y0, z ⫺ z0 2 represents any vector on the plane.
!
!
If! n ⫽ !ON ⫽ 1A, B, C 2 is a known normal to the plane, then the relationship,
!
n # P0P ⫽ 0 can be used to derive the equation of the plane, since n and any
vector on the plane are perpendicular.
!
!
n # P0P ⫽ 0
1A, B, C2 # 1x ⫺ x0, y ⫺ y0, z ⫺ z0 2 ⫽ 0
Ax ⫺ Ax0 ⫹ By ⫺ By0 ⫹ Cz ⫺ Cz0 ⫽ 0
Or, Ax ⫹ By ⫹ Cz ⫹ 1⫺Ax0 ⫺ By0 ⫺ Cz0 2 ⫽ 0
Since each of the quantities in the expression ⫺Ax0 ⫺ By0 ⫺ Cz0 are known,
we’ll replace this by D to make the equation simpler. The Cartesian equation of
the plane is thus, Ax ⫹ By ⫹ Cz ⫹ D ⫽ 0.
Cartesian Equation of a Plane
The Cartesian (or scalar) equation of a plane in R3 is of the form
!
!
Ax ⫹ By ⫹ Cz ⫹ D ⫽ 0 with normal n ⫽ 1A, B, C2. The normal n is a
nonzero vector perpendicular to all vectors in the plane.
EXAMPLE 1
Representing a plane by its Cartesian equation
!
The point A11, 2, 22 is a point on the plane with normal n ⫽ 1⫺1, 2, 6 2 .
Determine the Cartesian equation for this plane.
Solution
In determining the Cartesian equation for this plane, two different methods are
used, each of which will give the same answer.
Method 1:
Let P1x, y, z2 be any point on the plane.
!
Therefore, AP ⫽ 1x ⫺ 1, y! ⫺ 2, z ⫺ 2 2 represents any vector on the plane. Since
!
!
n ⫽ 1⫺1, 2, 6 2 and n # AP ⫽ 0,
1⫺1, 2, 6 2 # 1x ⫺ 1, y ⫺ 2, z ⫺ 22 ⫽ 0
⫺11x ⫺ 12 ⫹ 21y ⫺ 2 2 ⫹ 61z ⫺ 2 2 ⫽ 0
⫺x ⫹ 1 ⫹ 2y ⫺ 4 ⫹ 6z ⫺ 12 ⫽ 0
⫺x ⫹ 2y ⫹ 6z ⫺ 15 ⫽ 0
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Multiplying each side by ⫺1, x ⫺ 2y ⫺ 6z ⫹ 15 ⫽ 0.
Either of the equations ⫺x ⫹ 2y ⫹ 6z ⫺ 15 ⫽ 0 or x ⫺ 2y ⫺ 6z ⫹ 15 ⫽ 0 is a
correct equation for the plane, but usually we write the equation with integer
coefficients and with the coefficient of the x term being positive.
Method 2:
Since the required equation has the form, Ax ⫹ By ⫹ Cz ⫹ D ⫽ 0, where
!
n ⫽ 1A, B, C 2 ⫽ 1⫺1, 2, 6 2 , the direction numbers for the normal can be
substituted directly into the equation. This gives, ⫺x ⫹ 2y ⫹ 6z ⫹ D ⫽ 0 with D
to be determined. Since the point A11, 2, 2 2 is on the plane, it satisfies this equation.
Substituting the coordinates of this point into the equation gives
⫺112 ⫹ 212 2 ⫹ 6122 ⫹ D ⫽ 0, and thus D ⫽ ⫺15.
If D ⫽ ⫺15 is substituted into ⫺x ⫹ 2y ⫹ 6z ⫹ D ⫽ 0, the equation will be
⫺x ⫹ 2y ⫹ 6z ⫹ 1⫺15 2 ⫽ 0 or x ⫺ 2y ⫺ 6z ⫹ 15 ⫽ 0.
In finding the Cartesian equation for a plane, either method 1 or 2 can be used.
The Cartesian equation for a plane is simpler than either the vector or the
parametric form and is used most often in practice when dealing with planes.
EXAMPLE 2
Determining the Cartesian equation of a plane from three coplanar
points
Determine the Cartesian equation for the plane containing the points A1⫺1, 2, 5 2,
B13, 2, 42, and C1⫺2, ⫺3, 6 2 .
Solution
A normal to this plane
by calculating the cross product of the
! is determined
!
position vectors, AB and AC .
!
AB ⫽ 13 ⫺ 1⫺1 2, 2 ⫺ 2, 4 ⫺ 5 2 ⫽ 14, 0, ⫺12 and
!
AC ⫽ 1⫺2 ⫺ 1⫺1 2 , ⫺3 ⫺ 2, 6 ⫺ 5 2 ⫽ 1⫺1, ⫺5, 12.
!
!
Thus, AB ⫻ AC ⫽ 1011 2⫺1⫺12 1⫺52 , ⫺11⫺12 ⫺14 2 11 2, 41⫺5 2 ⫺10 2 1⫺1 2 2
⫽ 1⫺5, ⫺3, ⫺20 2 ⫽ ⫺115, 3, 20 2.
!
If we let P1x, y, z2 be any point on the plane, then AP ⫽ 1x ⫹ 1, y ⫺ 2, z ⫺ 5 2,
and if a normal to the plane is (5, 3, 20), then
15, 3, 20 2 # 1x ⫹ 1, y ⫺ 2, z ⫺ 5 2 ⫽ 0
5x ⫹ 5 ⫹ 3y ⫺ 6 ⫹ 20z ⫺ 100 ⫽ 0
After simplifying, the required equation for the plane is 5x ⫹ 3y ⫹ 20z ⫺ 101 ⫽ 0.
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A number
of
!
! observations can be made about this calculation. If we had used
for
AB! and BC as direction vectors,
! example, we would have found that
!
BC ⫽ 1⫺5, ⫺5, 22 and AB ⫻ BC ⫽ 14, 0, ⫺1 2 ⫻ 1⫺5, ⫺5, 22 ⫽ ⫺115, 3, 202 .
In finding the equation for a plane, it is possible to use any pair of direction
vectors on the plane to find a normal to the plane. Also, in finding the value
for D, if we had used the method of substitution, it would have been possible
to substitute any one of the three given points in the equation.
In the next example, we will show how to convert from vector or parametric form
to Cartesian form, and we will also show how to obtain the vector form of a plane
if given its Cartesian form.
EXAMPLE 3
Connecting the various forms of the equation of a plane
a. Find the Cartesian form of the plane whose equation in vector form is
!
r ⫽ 11, 2, ⫺1 2 ⫹ s11, 0, 2 2 ⫹ t1⫺1, 3, 4 2, s, t [ R.
b. Find the vector and parametric equations for the plane with Cartesian equation
x ⫺ 2y ⫹ 5z ⫺ 6 ⫽ 0.
Solution
a. In finding the Cartesian equation for this plane, two direction vectors are needed
so that a normal to the plane can be determined. The two given direction vectors
for this plane are 11, 0, 2 2 and 1⫺1, 3, 4 2. Their cross product,
11, 0, 2 2 ⫻ 1⫺1, 3, 4 2, is
1014 2⫺213 2, 21⫺1 2 ⫺114 2, 113 2⫺01⫺1 2 2 ⫽ ⫺312, 2, ⫺1 2.
A normal to the plane is 12, 2, ⫺12, and the Cartesian equation for the plane is
of the form 2x ⫹ 2y ⫺ z ⫹ D ⫽ 0. Substituting the point 11, 2, ⫺1 2 into this
equation gives 211 2 ⫹ 212 2⫺1⫺1 2 ⫹ D ⫽ 0 or D ⫽ ⫺7.
Therefore, the Cartesian equation for this plane is 2x ⫹ 2y ⫺ z ⫺ 7 ⫽ 0.
b. Method 1:
In finding the corresponding vector and parametric equations for a plane, the
equation of the plane is first converted to its parametric form. The simplest way
to do this is to choose any two of the variables and replace them with a
parameter. For example, if we substitute y ⫽ s and z ⫽ t and solve for x, we
would obtain x ⫺ 2s ⫹ 5t ⫺ 6 ⫽ 0 or x ⫽ 2s ⫺ 5t ⫹ 6.
This gives us the required parametric equations x ⫽ 2s ⫺ 5t ⫹ 6, y ⫽ s, and
z ⫽ t. The vector form of the plane can be found by rearranging the parametric
form.
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Therefore, 1x, y, z2 ⫽ 12s ⫺ 5t ⫹ 6, s, t2
1x, y, z2 ⫽ 16, 0, 0 2 ⫹ 12s, s, 0 2 ⫹ 1⫺5t, 0, t2
!
r ⫽ 16, 0, 0 2 ⫹ s12, 1, 0 2 ⫹ t1⫺5, 0, 1 2 , s, t [ R.
The parametric equations for this plane are x ⫽ 2s ⫺ 5t ⫹ 6, y ⫽ s, and
z ⫽ t, and the corresponding vector form is
!
r ⫽ 16, 0, 0 2 ⫹ s12, 1, 0 2 ⫹ t1⫺5, 0, 12, s, t [ R.
Check
This vector equation for the plane can be checked by converting to Cartesian form.
A normal to the plane is 12, 1, 0 2 ⫻ 1⫺5, 0, 1 2 ⫽ 11, ⫺2, 5 2 . The plane has the
form x ⫺ 2y ⫹ 5z ⫹ D ⫽ 0. If 16, 0, 02 is substituted into the equation to find D,
we find that 6 ⫺ 210 2 ⫹ 510 2 ⫹ D ⫽ 0 so D ⫽ ⫺6, and the equation is the given
equation x ⫺ 2y ⫹ 5z ⫺ 6 ⫽ 0.
Method 2:
We rewrite the given equation as x ⫽ 2y ⫺ 5z ⫹ 6. We are going to find the
coordinates of three points on the plane, and writing the equation in this way
allows us to choose integer values for y and z that will give an integer value
for x. The values in the table are chosen to make the computation as simple
as possible.
The following table shows our choices for y and z, along with the calculation for x.
y
z
x ⴝ 2y ⴚ 5z ⴙ 6
0
0
x ⫽ 2102 ⫺5102 ⫹ 6 ⫽ 6
A16, 0, 02
⫺1
0
x ⫽ 21⫺12 ⫺5102 ⫹ 6 ⫽ 4
B14, ⫺1, 02
⫺1
1
x ⫽ 21⫺12⫺5112 ⫹ 6 ⫽ ⫺1
C1⫺1, ⫺1, 12
Resulting Point
!
AB ⫽ 14 ⫺ 6, ⫺1 ⫺ 0, 0 ⫺ 0 2 ⫽ 1⫺2, ⫺1, 0 2 and
!
AC ⫽ 1⫺1 ⫺ 6, ⫺1 ⫺ 0, 1 ⫺ 0 2 ⫽ 1⫺7, ⫺1, 12 .
!
A vector equation is r ⫽ 16, 0, 0 2 ⫹ p1⫺2, ⫺1, 0 2 ⫹ q1⫺7, ⫺1, 1 2 , p, q [ R.
The corresponding parametric form is x ⫽ 6 ⫺ 2p ⫺ 7q, y ⫽ ⫺p ⫺ q, and
z ⫽ q.
Check
To check the correctness of these equations, the same procedure as shown in
Method 1 is used. This will give the identical Cartesian equation,
x ⫺ 2y ⫹ 5z ⫺ 6 ⫽ 0.
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When we considered lines in R2, we showed how to determine whether lines were
parallel or perpendicular. It is possible to use this same formula to determine
whether planes are parallel or perpendicular.
Parallel and Perpendicular Planes
Parallel Planes
n1
Perpendicular Planes
p1
p1
n2
n2
n1
p2
p2
!
!
1. If p1 and p2 are two perpendicular planes, with normals n1 and n2 , respectively,
!# !
their normals are perpendicular. 1i.e. n1 n2 ⫽ 0 2
!
!
2. If p1 and p2 are two parallel planes, with normals n1 and n2 , respectively,
!
!
their normals are parallel. 1i.e. n1 ⫽ kn2 2
EXAMPLE 4
Reasoning about parallel and perpendicular planes
a. For p1 : 2x ⫺ 3y ⫹ z ⫺ 1 ⫽ 0 and p2 : 4x ⫺ 3y ⫺ 17z ⫽ 0, show that these
planes are perpendicular.
b. Show that the planes p3 : 2x ⫺ 3y ⫹ 2z ⫺ 1 ⫽ 0 and p4 : 2x ⫺ 3y ⫹ 2z ⫺ 3 ⫽ 0
are parallel but not coincident.
Solution
!
!
a. For p1, n1 ⫽ 12, ⫺3, 1 2 and for p2 : n2 ⫽ 14, ⫺3, ⫺172 .
! !
n1 # n2 ⫽ 12, ⫺3, 12 # 14, ⫺3, ⫺172
⫽ 214 2 ⫺ 31⫺3 2 ⫹ 11⫺17 2
⫽ 8 ⫹ 9 ⫺ 17 ⫽ 0
! !
Since n1 # n2 ⫽ 0 the two planes are perpendicular to each other.
!
!
b. For p3 and p4, n3 ⫽ n4 ⫽ 12, ⫺3, 2 2, so the planes are parallel. Because the
planes have different constants, i.e., ⫺1 and ⫺3, the planes are not coincident.
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In general, if planes are coincident, it means that the planes have equations
that are scalar multiples of each other. For example, the two planes
2x ⫺ y ⫹ z ⫺ 13 ⫽ 0 and ⫺6x ⫹ 3y ⫺ 3z ⫹ 39 ⫽ 0 are coincident
because ⫺6x ⫹ 3y ⫺ 3z ⫹ 39 ⫽ ⫺312x ⫺ y ⫹ z ⫺ 13 2.
It is also possible to find the angle between intersecting planes using their
normals and the dot product formula for calculating the angle between two
vectors. The angle between two planes is the same as the angle between their
normals.
Angle Between Intersecting Planes
p1
n2
u u n1
p2
u
!
!
The angle u between two planes, p1 and p2, with normals of n! 1 and
n2 ,
!
n1 # n2
respectively, can be calculated using the formula, cos u ⫽ ! ! .
0n 1 0 0n 2 0
EXAMPLE 5
Calculating the angle formed between two intersecting planes
Determine the angle between the two planes p1 : x ⫺ y ⫺ 2z ⫹ 3 ⫽ 0 and
p2 : 2x ⫹ y ⫺ z ⫹ 2 ⫽ 0.
Solution
!
!
For p1 : n1 ⫽ 11, ⫺1, ⫺2 2 and for p2 : n2 ⫽ 12, 1, ⫺1 2. Since
!
0n 1 0 ⫽ V112 2 ⫹ 1⫺12 2 ⫹ 1⫺2 2 2 ⫽ V6 and
!
0n 2 0 ⫽ V12 2 2 ⫹ 112 2 ⫹ 1⫺1 2 2 ⫽ V6,
cos u ⫽
11, ⫺1, ⫺22 # 12, 1, ⫺12
兹6 兹6
2⫺1⫹2
⫽
6
1
⫽ .
2
Therefore, the angle between the two planes is 60°. Normally, the angle between
planes is given as an acute angle, but it is also correct to give the angle as 120°.
44
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IN SUMMARY
• The Cartesian equation of a line in R3 is Ax ⫹ By ⫹ Cz ⫹ D ⫽ 0 where
!
!
!
!
n ⫽ 1A, B, C2 is a normal to the plane, and n ⫽ a ⫻ b .
Need to Know
!
!
• Two planes are parallel if and only if n1 ⫽ kn2 .
!
!
• Two planes are perpendicular if and only if n1 # n2 ⫽ 0.
!
!
n1 # n2
⫺1
• The angle between two planes is u ⫽ cos a !
! b.
0n1 0 0n2 0
Exercises
PART A
1. A plane is defined by the equation, x ⫺ 7y ⫺ 18z ⫽ 0.
a. What is a normal vector to this plane?
b. Explain how you know this plane passes through the origin.
c. Write the coordinates of three points on this plane.
2. A plane is defined by the equation 2x ⫺ 5y ⫽ 0.
a. What is a normal vector to this plane?
b. Explain how you know this plane passes through the origin.
c. Write the coordinates of three points on this plane.
3. A plane is defined by the equation x ⫽ 0.
a. What is a normal vector to this plane?
b. Explain how you know this plane passes through the origin.
c. Write the coordinates of three points on this plane.
!
4. a. A plane is determined by a normal, n ⫽ 115, 75, ⫺105 2 , and passes through
the origin. Write the Cartesian equation for this plane where the normal is in
reduced form.
!
1 3 7
b. If a plane has a normal of n ⫽ Q⫺2, 4, 16R and passes through the origin,
determine the Cartesian equation of this plane.
PART B
!
5. A plane is determined by a normal, n ⫽ 11, 7, 5 2 , and contains the point
P1⫺3, 3, 5 2 . Determine a Cartesian equation for this plane using the two
methods shown in Example 1.
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6. The three noncollinear points P1⫺1, 2, 12 , Q13, 1, 42, and R1⫺2, 3, 5 2 lie on
a plane.
!
!
a. Using PQ and QR as position vectors and the point R1⫺2, 3, 5 2, determine
the Cartesian equation for this plane.
!
!
b. Using QP and PR as position vectors and the point P1⫺1, 2, 1 2, determine
the Cartesian equation for this plane.
c. Explain why the two equations must be the same.
7. Determine the Cartesian equation of the plane that contains the points
A1⫺2, 3, 12, B13, 4, 5 2, and C11, 1, 0 2.
!
8. The line with vector equation r ⫽ 12, 0, 1 2 ⫹ s1⫺4, 5, 5 2 , s [ R lies on the
plane p, as does the point P11, 3, 02 . Determine the Cartesian equation of p.
9. Determine unit vectors that are normal to each of the following planes.
a. 2x ⫹ 2y ⫺ z ⫺ 1 ⫽ 0
b. 4x ⫺ 3y ⫹ z ⫺ 3 ⫽ 0
c. 3x ⫺ 4y ⫹ 12z ⫺ 1 ⫽ 0
!
10. A plane contains the point A12, 2, ⫺1 2 and the line r ⫽ 11, 1, 5 2 ⫹ s12, 1, 3 2 ,
s [ R. Determine the Cartesian equation of this plane.
A
C
11. Find the Cartesian equation of the plane containing the point 1⫺1, 1, 02 and
perpendicular to the line joining the points 11, 2, 1 2 and 13, ⫺2, 0 2.
12. a. Explain the process you would use to determine the angle formed between
two intersecting planes.
b. Determine the angle between the planes x ⫺ z ⫹ 7 ⫽ 0 and
2x ⫹ y ⫺ z ⫹ 8 ⫽ 0.
13. a. Find the angle between the planes x ⫹ 2y ⫺ 3z ⫺ 4 ⫽ 0 and
x ⫹ 2y ⫺ 1 ⫽ 0.
b. Determine the Cartesian equation of the plane through the point P11, 2, 1 2
z⫹4
x⫺3 y⫹1
⫽
⫽
.
and which is perpendicular to the line
⫺2
3
1
14. a. What is the value of k so that the planes 4x ⫹ ky ⫺ 2z ⫹ 1 ⫽ 0 and
2x ⫹ 4y ⫺ z ⫹ 4 ⫽ 0 are parallel? perpendicular?
b. Can these two planes ever be coincident? Explain your answer.
T
15. Find the Cartesian equation of the plane passing through the points 11, 4, 5 2
and 13, 2, 12 and which is perpendicular to the plane 2x ⫺ y ⫹ z ⫺ 1 ⫽ 0.
PART C
16. Find an equation of the plane that is perpendicular to the plane x ⫹ 2y ⫹ 4 ⫽ 0,
contains the origin, and where the normal makes an angle of 30° with the z-axis.
46
8.5
17. Find the equation of the plane that lies between the points 1⫺1, 2, 4 2 and
13, 1, ⫺42 and that is equidistant from them.
T H E C A RT E S I A N E Q UAT I O N O F A P L A N E
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Section 8.6—Sketching Planes in R3
In previous sections, we developed techniques for finding the equation of planes
in both vector and Cartesian form. In this section, we examine how to sketch a
plane where the equation is in Cartesian form. In the graphing of planes in R3,
many of the same techniques used for graphing a line in R2 will be used.
An important first observation is that, if we are given an equation such as x 5 4
and are asked to find its related graph, then a different graph is produced
depending on the dimension in which we are working. In the following diagram,
we show three different graphs for x 5 4.
Different interpretations of the graph with equation x 5 4.
z
y x= 4
R
–1 0 1 2 3 4
x=4
x=4
R2
Real Number Line, R
O(0, 0)
R3
(4, s, t)
(4, s), s[R
(4, 0)
x
y
(4, 0, 0)
x
1. On the real number line, this refers to a point at x 5 4.
2. In R2, this equation represents a line parallel to the y-axis and 4 units to its
right.
3. In R3, the equation x 5 4 represents a plane that intersects the x-axis at
(4, 0, 0) and is 4 units in front of the plane formed by the y- and z-axes.
We can see from this example that the equation x 5 4 gives a different graph
depending upon whether it is drawn on the number line, in R2, or in R3.
In the following, the graphing of Ax 1 By 1 Cz 1 D 5 0 in R3 is considered for
different cases.
Case 1
a. Two of A, B, or C equal zero and D 5 0.
In this case, the resulting equations will be of the form, x 5 0, y 5 0,
or z 5 0. If x 5 0, for example, this equation represents the yz-plane,
since every point on this plane has an x-value equal to 0. Similarly,
y 5 0 represents the xz-plane, and z 5 0 represents the xy-plane.
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b. Two of A, B, or C are equal to zero and D Þ 0.
Case 1
If two of the three coefficients are equal to zero, the resulting
equations would be of the form x 5 a, y 5 b, or z 5 c. The following
examples show that these equations represent planes parallel to the
yz-, xz- and xy-planes, respectively.
EXAMPLE 1
Representing the graphs of planes in R3 whose Cartesian equations
involve 1 variable
Draw the planes with equations p1 : x 5 5, p2 : y 5 6, and p3 : z 5 6.
Solution
z
z
z
p2
(0, 0, 6)
p3
p3: z = 6
p1
(0, 6, 0)
y
y
y
(5, 0 ,0)
x
p1: x = 5
x
x
p2: y = 6
A description of the planes in Example 1 is given in the following table.
Plane
Description
Generalization
p1 : x 5 5
A plane parallel to the yz-plane
crosses the x-axis at (5, 0, 0). This
plane has an x-intercept of 5.
A plane with equation x 5 a is parallel
to the yz-plane and crosses the x-axis
at the point 1a, 0, 02. The plane x 5 0
is the yz-plane.
p2 : y 5 6
A plane parallel to the xz-plane
crosses the y-axis at (0, 6, 0). This
plane has a y-intercept of 6.
A plane with equation y 5 b is parallel
to the xz-plane and crosses the y-axis
at the point 10, b, 02. The plane y 5 0
is the xz-plane.
p3 : z 5 6
A plane parallel to the xy-plane
crosses the x-axis at the point
(0, 0, 6). This plane has a
z-intercept of 6.
A plane of the form z 5 c is parallel to
the xy-plane and crosses the z-axis at
the point 10, 0, c2. The plane z 5 0
is the xy-plane.
Case 2
a. One of A, B, or C equals zero and D 5 0.
In this case, the resulting equations would have the form
Ax 1 By 5 0, Ax 1 Cz 5 0, or By 1 Cz 5 0. The following
example demonstrates how to graph an equation of this type.
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Representing the graph of a plane in R3 whose Cartesian equation
involves 2 variables, D 5 0
Sketch the plane having the equation p1 : 2x 2 y 5 0.
Solution
For p1:2x 2 y 5 0, we note that the origin O10, 0, 02 lies on this plane, and
it also contains the z-axis. We can see that p1 contains the z-axis because, if
it is written in the form 2x 2 y 1 0z 5 0, (0, 0, t) is on this plane because
210 220 1 01t2 5 0. Since (0, 0, t), t [ R, represents any point on the z-axis,
this means that the plane contains the z-axis.
In addition, the plane cuts the xy-plane along the line 2x 2 y 5 0. All that is
necessary to graph this line is to select a point on the xy-plane that satisfies this
equation and join that point to the origin. Since the point with coordinates A11, 2, 0 2
satisfies this equation, we draw the parallelogram determined by the z-axis and the
line joining O10, 0, 0 2 to A11, 2, 02, and we have a sketch of the plane 2x 2 y 5 0.
z
p1
(0, 2, 0)
O(0, 0, 0)
y
(1, 0, 0) 2x– y =0
A(1, 2, 0)
x
EXAMPLE 3
Describing planes whose Cartesian equations involve two variables,
D50
Write a description of the planes p1 : 2x 2 z 5 0 and p2 : y 1 2z 5 0.
Solution
These equations can be written as: p1 : 2x 1 0y 2 z 5 0 and
p2 : 0x 1 y 1 2z 5 0.
Using the same reasoning as above, this implies that p1 contains the origin and
the y-axis and cuts the xz-plane along the line with equation 2x 2 z 5 0.
Similarly, p2 contains the origin and the x-axis and cuts the yz-plane along the
line with equation y 1 2z 5 0.
Case 2
NEL
b. One of A, B or C is equal to zero and D Þ 0.
In the case in which one of the coefficients equals zero, and D Þ 0,
the resulting equations can be written as Ax 1 By 1 D 5 0,
Ax 1 Cz 1 D 5 0, and By 1 Cz 1 D 5 0.
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Graphing planes whose Cartesian equations involve two variables, D Þ 0
Sketch the plane having the equation p1 : 2x 2 5y 2 10 5 0.
Solution
In considering p1, it is best to write this equation as 2x 2 5y 5 10 so that we can
easily calculate its intercepts.
x-intercept: We calculate the x-intercept for this plane in exactly the same way that
we would calculate the x-intercept for the line 2x 2 5y 5 10. If we let
y 5 0, then 2x 2 510 2 5 10 or x 5 5. This means that the plane has
an x-intercept of 5 and that it crosses the x-axis at 15, 0, 0 2 .
y-intercept: In calculating the y-intercept, we let x 5 0. Thus,
25y 5 10, y 5 22. This means that the plane has a y-intercept
of 22 and it crosses the y-axis at 10, 22, 02 .
To complete the analysis for this plane, we write the equation as
2x 2 5y 1 0z 5 10. If this plane did cross the z-axis, it would do so at a point
where x 5 y 5 0. Substituting these values into the equation, we obtain
210 2 2 510 2 1 0z 5 10 or 0z 5 10. This implies that this plane has no z intercept
because there is no value of z that will satisfy this equation. Thus, the plane passes
through the points (5, 0, 0) and 10, 22, 02 and is parallel to the z-axis. The plane
is sketched in the diagram below. Possible direction vectors for this plane are
!
!
m1 5 15 2 0, 0 2 122 2 , 0 2 0 2 5 15, 2, 02 and m2 5 10, 0, 1 2 .
z
p1
2x – 5y = 10
(0, – 2, 0)
y
x
Case 3
(5, 0, 0)
a. None of A, B, or C is equal to zero and D 5 0.
This represents an equation of the form, Ax 1 By 1 Cz 5 0, which
is a plane most easily sketched using the fact that a plane is uniquely
determined by three points. The following example illustrates this
approach.
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Graphing planes whose Cartesian equations involve three variables,
D50
Sketch the following plane: p1 : x 1 3y 2 z 5 0.
Solution
Since there is no constant in this equation, the point 10, 0, 0 2 is on the plane. To
draw a sketch of this plane, we require two other points. We first find a point on
the xy-plane and a second one on the xz-plane.
Point on xy-plane: Any point on the xy-plane has z 5 0. If we first substitute
z 5 0 into x 1 3y 2 z 5 0, then x 1 3y 2 0 5 0, or x 1 3y 5 0, which means
that the given plane cuts the xy-plane along the line x 1 3y 5 0. Using this
equation, we can now select convenient values for x and y to obtain the
coordinates of a point on this line. The easiest values are x 5 23 and y 5 1,
implying that the point A123, 1, 02 is on the plane.
Point on xz-plane: Any point on the xz-plane has y 5 0. If we substitute y 5 0
into x 1 3y 2 z 5 0, then x 1 310 2 2 z 5 0, or x 5 z, which means that the
given plane cuts the xz-plane along the line x 5 z. As before, we choose
convenient values for x and z. The easiest values are x 5 z 5 1 implying that
B11, 0, 12 is a point on the plane.
Since three points determine a plane, we locate these points in R3 and form the
related triangle. This triangle OAB represents part of the required plane.
z
A(–3, 1, 0)
B(1, 0, 1)
L1: x –z=0
p1
L1: x +3y = 0
y
O(0, 0, 0)
x
Case 3
b. None of the coefficients equals zero and D Þ 0.
This represents the plane with equation Ax 1 By 1 Cz 1 D 5 0, and
this plane is most easily sketched by finding its intercepts. Since we
know that three points determine a plane, knowing these three
intercepts will allow us to graph the plane.
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Graphing planes whose Cartesian equations involve three variables,
DÞ0
Sketch the plane having equation p1 : 2x 1 3y 2 z 5 4.
Solution
In sketching this plane, we calculate the coordinates of the points where this plane
intersects each of the three coordinate axes (i.e., we determine the three intercepts
for the plane). This is accomplished by setting 2 of the 3 variables equal to zero
4
and solving for the remaining variable. The x-, y-, and z-intercepts are 2, 3, and 24,
respectively. These three points form a triangle that forms part of the required
plane.
z
O(0, 0, 0)
(2, 0, 0)
(0, 4 , 0)
3
p1
y
2x +3y–z=4
(0, 0, –4)
x
EXAMPLE 7
Reasoning about direction vectors of planes
Determine two direction vectors for the planes, p1 : 3x 1 4y 5 12 and
p2 : x 2 y 2 5z 5 0.
Solution
The plane p1 : 3x 1 4y 5 12 crosses the x-axis at the point 14, 0, 0 2 and the
y-axis at the point 10, 3, 0 2. One direction vector is thus
!
m 1 5 14 2 0, 0 2 3, 0 2 0 2 5 14, 23, 0 2 . Since this plane can be written as
3x 1 4y 1 0z 5 12, this implies that it does not intersect the z-axis, and
!
therefore has m 2 5 10, 0, 12 as a second direction vector.
The plane p2 : x 2 y 2 5z 5 0 passes through O10, 0, 02 and cuts the xz-plane
along the line x 2 5z 5 0. Convenient choices for x and z are 5 and 1,
respectively. This means that A15, 0, 1 2 is on p2. Similarly the given plane cuts
the xy-plane along the line x 2 y 5 0. Convenient values for x and y are 1 and 1.
This means that B11, 1, 0 2 is on p2.
!
Possible direction vectors for p2 are m 1 5 15 2 0, 0 2 0, 1 2 02 5 15, 0, 1 2 and
!
m 2 5 11, 1, 02.
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IN SUMMARY
Key Idea
• A sketch of a plane in R3 can be created by using combination of points and
lines that help define the plane.
Need to Know
• To sketch the graph of a plane consider each of the following cases as it
relates to the Cartesian equation Ax 1 By 1 Cz 1 D 5 0
Case 1: The equation contains one variable
a.: Two of the coefficients (i.e., two of A, B, or C ) are equal to zero, D 5 0.
These are the three coordinate planes xy-plane, xz-plane and yz-plane.
b.: Two of the coefficients (i.e., two of A, B, or C ) are equal to zero, D Þ 0.
These are parallel to the three coordinate planes.
Case 2: The equation contains two variables
a.: One of the coefficients (i.e., one of A, B, or C ) is equal to zero, D 5 0.
Find a point with missing variable 0. Join to (0, 0, 0) and draw a plane
containing the missing variable axis and the above-found point.
b.: One of the coefficients (i.e., one of A, B, or C ) is equal to zero, D Þ 0.
Find the two intercepts and draw a plane parallel to the missing variable axis.
Case 3: The equation contains three variables
a.: None of the coefficients equals zero (i.e., none of A, B, or C ) is equal to
zero, D 5 0.
Determine two points with 10, 0, 0 2 and draw the plane through the points.
b.: None of the coefficients (i.e., none of A, B, or C ) is equal to zero, D Þ 0.
Find the three intercepts and draw a plane through these three points.
Exercise 8.6
C
PART A
1. Describe each of the following planes in words.
a. x 5 22
b. y 5 3
c. z 5 4
2. For the three planes given in question 1, what are coordinates of their point of
intersection?
3. On which of the planes p1: x 5 5 or p2 : y 5 6 could the point P15, 23, 23 2
lie? Explain.
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PART B
4. Given that x 2 2 1 5 1x 2 12 1x 1 12 , sketch the two graphs associated with
x 2 2 1 5 0 in R2 and R3.
5. a. For the following three planes, state the x-, y-, and z-intercepts for each.
b. State two direction vectors for each plane.
i. p1 : 2x 1 3y 5 18
ii. p2 : 3x 2 4y 1 5z 5 120
iii. p3 : 13y 2 z 5 39
6. For the plane with equation p : 2x 2 y 1 5z 5 0, determine the following.
a. the coordinates of three points on this plane
b. the equation of the line where this plane intersects the xy-plane
c. a sketch of this plane
7. Name the three planes that the equation xyz 5 0 represents in R3.
K
8. For each of the following equations, draw a sketch of the corresponding
plane.
a. p1 : 4x 2 y 5 0
c. p3 : z 5 4
b. p2 : 2x 1 y 2 z 5 4
d. p4 : y 2 z 5 4
9. a. Write the following expression in factored form, xy 1 2y 5 0.
b. Draw a sketch of the lines corresponding to this expression in R2.
c. Draw a sketch of the planes corresponding to this expression in R3.
10. For each given equation, draw a sketch of the corresponding plane.
a. 2x 1 2y 1 z 2 4 5 0
b. 3x 2 4z 5 12
c. 5y 2 15 5 0
T
PART C
11. It is sometimes useful to be able to write an equation of a plane in terms of its
intercepts. If a, b, and c represent the x-, y-, and z-intercepts, respectively,
then the resulting equation is, ax 1 by 1 cz 5 1.
a. Determine the equation of the plane having x-, y-, and z-intercepts of
3, 4, and 6, respectively.
b. Determine the equation of the plane having x- and z-intercepts of 5 and
–7, respectively, and which is parallel to the y-axis.
c. Determine the equation of the plane having no x- or y-intercept but with a
z-intercept of 8.
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CAREER LINK WRAP-UP Investigate and Apply
CHAPTER 8: COMPUTER PROGRAMMING WITH VECTORS
A computer programmer is designing a 3-D space game. She wants to have an
asteroid fly past a spaceship along the path of vector m, collide with another
asteroid, and be deflected along vector path l. The spaceship is treated as the
origin and is travelling along vector d.
z
l(5, 20, 20)
y
d(10, 10, 10) m(15, 25, 5)
(0, 0, 0)
x
a. Find the vector and parametric equations of the line determined by vector m.
b. Add vectors m and l. Determine if an object whose path is along vector d will
cross through the path l of the asteroid, resulting in a possible collision
between the spaceship and the asteroid.
c. Draw vector h, normal to l, that passes through the origin and is
perpendicular to l.
d. Draw vector j parallel to h that passes through (10, 10, 10). Find the vector
and parametric equations of the line determined by vector j.
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Key Concepts Review
In Chapter 8 you examined how the algebraic description of straight lines could
be represented using vectors in both two and
three dimensions. The form of
!
!
!
the vector equation of a line, r ⫽ r0 ⫹ t d , is the same whether the line lies in
a two dimensional plane or in a three dimensional space. The following table
summarizes the various forms of the equation of a line where the point
1x0, y0, 1z0 22 lies on the line and 1a, b, 1c22 is a direction vector of the line:
R2
R3
Vector equation
1x, y2 ⫽ 1x0, y0 2 ⫹ t 1a, b2
1x, y, z2 ⫽ 1x0, y0, z0 2 ⫹ t 1a, b, c2
Parametric equations
x ⫽ x0 ⫹ at, y ⫽ y0 ⫹ bt
x ⫽ x0 ⫹ at, y ⫽ y0 ⫹ bt, z ⫽ z0 ⫹ ct
Symmetric equations
y ⫺ y0
x ⫺ x0
⫽
a
b
y ⫺ y0
x ⫺ x0
z ⫺ z0
⫽
⫽
a
b
c
Cartesian equation
Ax ⫹ BY ⫹ C ⫽ 0
not applicable
This concept was then extended to planes in R3. The following table summarizes the
various forms of the equation of a plane where 1x0, y0, z0 2 is a point that lies on the
plane and 1a1, a2, a3 2 and 1b1, b2, b3 2 are direction vectors that also lie on the plane:
R3
Vector equation
1x, y, z2 ⫽ 1x0, y0, z0 2 ⫹ s 1a1, a2, a3 2 ⫹ t 1b1, b2, b3 2
Parametric equations
x ⫽ x0 ⫹ sa1 ⫹ tb1, y ⫽ y0 ⫹ sa2 ⫹ tb2, z ⫽ z0 ⫹ sa3 ⫹ tb3
Cartesian equation
Ax ⫹ BY ⫹ Cz ⫹ D ⫽ 0
You also saw that when lines and planes intersect, that angles are formed
between them. Both lines and planes have normals which are vectors that run
perpendicular to the respective line or plane. The size of these angles can be
determined using these normal vectors and the following formula:
! !
n1 # n2
cos u ⫽ ! !
0n1 0 0n2 0
Sketching the graphs of planes in R3 can be accomplished by examining the
scalar equation of the plane. Determine if the equation contains one, two or three
variables and whether it contains a constant. This information helps to narrow
down which case you need to consider to sketch its graph. Once the specific case
is determined (see the In Summary table in Section 8.6) determine appropriate
points and lines to help you sketch a representation of the plane.
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KEY CONCEPTS REVIEW
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Review Exercise
1. Determine vector and parametric equations for the plane that contains the
points A11, 2, ⫺1 2, B12, 1, 1 2 , and C13, 1, 42 .
2. In question 1, there are a variety of different answers possible, depending
upon the points and direction vectors chosen. Determine two Cartesian
equations for this plane using two different vector equations and verify that
these two equations are identical.
3. a. Determine the vector, parametric, and symmetric equations for the line
passing through A1⫺3, 2, 8 2 and B14, 3, 9 2.
b. Determine the vector and parametric equations for the plane containing the
points A1⫺3, 2, 8 2, B14, 3, 9 2 , and C1⫺2, ⫺1, 3 2 .
c. Explain why a symmetric equation cannot exist for a plane.
4. Determine vector, parametric, and symmetric equations for the line passing
through the point A17, 1, ⫺22 and perpendicular to the plane with equation
2x ⫺ 3y ⫹ z ⫺ 1 ⫽ 0.
5. Determine the Cartesian equation for each of the following planes.
!
a. the plane through the point P10, 1, ⫺22 with normal n ⫽ 1⫺1, 3, 3 2
b. the plane through the points 13, 0, 1 2 and 10, 1, ⫺12 , and perpendicular to
the plane with equation x ⫺ y ⫺ z ⫹ 1 ⫽ 0
c. the plane through the points 11, 2, 1 2 and 12, 1, 4 2 and parallel to the
x-axis
6. Determine the Cartesian equation of the plane passing through the origin and
!
which contains the line r ⫽ 13, 7, 1 2 ⫹ t12, 2, 3 2, t [ R.
7. Find the vector and parametric equations for the plane parallel to the yz-plane
which contains the point A1⫺1, 2, 12.
8. Determine the Cartesian equation of the plane that contains the line
!
r ⫽ 12, 3, 2 2 ⫹ t11, 1, 42, t [ R, and the point 14, ⫺3, 22 .
9. Determine the Cartesian equation for the plane that contains the lines
!
L 1 : r ⫽ 14, 4, 5 2 ⫹ t15, ⫺4, 6 2, s [ R, and
!
L 2 : r ⫽ 14, 4, 5 2 ⫹ s12, ⫺3, ⫺4 2 , s [ R.
10. Determine an equation for the line perpendicular to the plane
3x ⫺ 2y ⫹ z ⫽ 1 and that passes through 12, 3, ⫺3 2. Give your answer in
vector, parametric, and symmetric form.
11. A plane has 3x ⫹ 2y ⫺ z ⫹ 6 ⫽ 0 as its Cartesian equation. Determine
vector and parametric equations for this plane.
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12. Determine an equation for the line that has the same x-and z-intercepts as the
plane with equation 2x ⫹ 5y ⫺ z ⫹ 7 ⫽ 0. Give your answer in vector,
parametric, and symmetric form.
13. Determine the vector, parametric, and Cartesian forms of the equation for the
!
plane containing the lines L 1 : r ⫽ 13, ⫺4, 1 2 ⫹ s11, ⫺3, ⫺52, s [ R, and
!
L 2 : r2 ⫽ 17, ⫺1, 02 ⫹ t12, ⫺6, ⫺102, t [ R.
14. Draw sketches for each of the following planes.
a. p1 : 2x ⫹ 3y ⫺ 6z ⫺ 12 ⫽ 0
b. p2 : 2x ⫹ 3y ⫺ 12 ⫽ 0
c. p3 : x ⫺ 3z ⫺ 6 ⫽ 0
d. p4 : y ⫺ 2z ⫺ 4 ⫽ 0
e. p5 : 2x ⫹ 3y ⫺ 6z ⫽ 0
15. Determine the vector, parametric, and Cartesian equations for each of the
following planes:
a. passing through the points P11,
! ⫺2, 52 and
! Q13, 1, 2 2! and parallel to the
!
line with equation L : r ⫽ 2ti ⫹ 14t ⫹ 32 j ⫹ 1t ⫹ 1 2 k , t [ R
b. containing the point A11, 1, 2 2 and perpendicular to the line joining the
points B12, 1, ⫺62 and C1⫺2, 1, 5 2
c. through the points 14, 1, ⫺1 2 and 15, ⫺2, 4 2 and parallel to the z-axis
d. through the points 11, 3, ⫺5 2 , 12, 6, 4 2 , and 13, ⫺3, 3 2
!
16. Show that L 1 : r ⫽ 11, 2, 3 2 ⫹ s1⫺3, 5, 212 ⫹ t10, 1, 32 , s, t [ R, and
!
L 2 : r ⫽ 11, ⫺1, ⫺6 2 ⫹ u11, 1, 12 ⫹ v12, 5, 11 2, u, v [ R, are equations for
the same plane.
!
17. The two lines L 1 : r ⫽ 1⫺1, 1, 0 2 ⫹ s12, 1, ⫺12 , s [ R, and
!
L 2 : r ⫽ 12, 1, 22 ⫹ t12, 1, ⫺12, t [ R, are parallel but do not coincide. The
point with coordinates A15, 4, ⫺3 2 is on L 1. Determine the coordinates of a
!
point B on L 2 such that AB is perpendicular to L 2.
18. Write a brief description for the following planes.
a. p1 : 2x ⫺ 3y ⫽ 6
b. p2 : x ⫺ 3z ⫽ 6
c. p3 : 2y ⫺ z ⫽ 6
19. a. Which of the following points lies on the line x ⫽ 2t, y ⫽ 3 ⫹ t, z ⫽ 1 ⫹ t?
A12, 4, 22
B1⫺2, 2, 1 2
C14, 5, 2 2
D16, 6, 2 2
b. If the point 1a, b, ⫺32 lies on the line, determine the values of a and b.
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20. Calculate the angles that are formed by the intersection of each of the
following pairs of lines.
a. L1 :
y⫺3
1⫺y
x⫺1
x⫺2
⫽
and L 2 :
⫽
1
5
2
3
b. y ⫽ 4x ⫹ 2 and y ⫽ ⫺x ⫹ 3
c. L1: x ⫽ ⫺1 ⫹ 3t, y ⫽ 1 ⫹ 4t, z ⫽ ⫺2t and
L 2: x ⫽ ⫺1 ⫹ 2s, y ⫽ 3s, z ⫽ ⫺7 ⫹ s
d. L1: 1x, y, z2 ⫽ 14, 7,⫺1 2 ⫹ t14, 8,⫺42 and
L 2: 1x, y, z2 ⫽ 11, 5, 4 2 ⫹ s1⫺1, 2 ,3 2
21. Calculate the angles that are formed by the intersection of each of the
following pairs of planes.
a. 2x ⫹ 3y ⫺ z ⫹ 9 ⫽ 0 and x ⫹ 2y ⫹ 4 ⫽ 0
b. x ⫺ y ⫺ z ⫺ 1 ⫽ 0 and 2x ⫹ 3y ⫺ z ⫹ 4 ⫽ 0
22. a. Which of the following lines is parallel to the plane 4x ⫹ y ⫺ z ⫺ 10 ⫽ 0?
b. Do any of the lines lie in the plane in part a?
!
i. r ⫽ 13, 0, 2 2 ⫹ t11,⫺2, 22
ii. x ⫽ ⫺3t, y ⫽ ⫺5 ⫹ 2t, z ⫽ ⫺10t
y⫹6
x⫺1
z
⫽
⫽
iii.
4
⫺1
1
23. Does the point (4, 5, 6) lie in the plane
1x, y, z2 ⫽ 14, 1, 6 2 ⫹ p13, ⫺2, 12 ⫹ q1⫺6, 6, ⫺1 2?
Support your answer with the appropriate calculations.
24. Determine the parametric equations of the plane that contains the two parallel
lines
L1: 1x, y, z 2 ⫽ 12, 4, 12 ⫹ t13, ⫺1, 1 2
L2: 1x, y, z 2 ⫽ 11, 4, 42 ⫹ t1⫺6, 2, ⫺22
25. Explain why the vector equation of a plane has two parameters while the
vector equation of a line has only one?
26. Explain why any plane with a vector equation of the form
1x, y, z2 ⫽ 1a, b, c2 ⫹ s1d, e, f 2 ⫹ t1a, b, c2 will always pass through the origin.
27. a. Explain why the three points 12, 3, ⫺12, 18, 5, ⫺52 and 1⫺1, 2, 1 2 do not
determine a plane.
!
b. Explain why the line r ⫽ 14, 9, ⫺32 ⫹ t11, ⫺4, 22 and the point 18, ⫺7, 5 2
do not determine a plane.
28. Find a formula for the scalar equation of a plane in terms of a, b, and c, where
a, b, and c are the x-intercept, the y-intercept and the z-intercept of a plane,
respectively.
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29. Determine the scalar equation of the plane that has normal vector 16, ⫺5, 12 2
and passes through the point 15, 8, ⫺3 2.
30. A plane passes through the points A11, ⫺3, 2 2, B1⫺2, 4, ⫺2 2 and C13, 2, 1 2.
a. Determine a vector equation of the plane.
b. Determine a set of parametric equations of the plane.
c. Determine the scalar equation of the plane.
d. Determine if the point 13, 5, ⫺4 2 lies on the plane.
31. Determine the scalar equation of the plane that is parallel to the plane
4x ⫺ 2y ⫹ 5z ⫺ 10 ⫽ 0 and that passes through each point.
a. 10, 0, 0 2
b. 1⫺1, 5, ⫺1 2
c. 12, ⫺2, 22
32. Show that the following pairs of lines intersect. Determine the coordinates of
the point of intersection, and the angles formed by the lines.
a. L1: x ⫽ 5 ⫹ 2t and L2: x ⫽ 23 ⫺ 2s
y ⫽ ⫺3 ⫹ t
y⫽6⫺s
y⫹1
y⫺2
x⫹3
x⫺6
⫽
⫽
b. L1:
and L2:
3
4
3
⫺2
33. Determine the vector equation, parametric equations and, if possible, the
symmetric equation of the line that passes through P11, 3, 52
a. with direction vector 1⫺2, ⫺4, ⫺102
b. and the point Q1⫺7, 9, 3 2
c. and is parallel to the line that passes through R14, 8, ⫺5 2 and S1⫺2, ⫺5, 9 2
d. and parallel to the x-axis
e. perpendicular to the line 1x, y, z2 ⫽ 11, 0, 5 2 ⫹ t1⫺3, 4, ⫺6 2
f. perpendicular to the plane determined by the point A14, 2, 1 2, B13, ⫺4, 2 2
and C1⫺3, 2, 1 2
34. Determine the scalar equation of the plane
a. containing the point P1⫺2, 6, 12 and with a normal vector 12, ⫺4, 5 2
b. containing the point P1⫺2, 0, 62 and the line
y⫹2
x⫺4
1
⫽ ⫺5
⫽z⫺
3
2
c. containing the point P13, 3, 3 2 and parallel to the xy-plane
d. containing the point P1⫺4, 2, 4 2 and parallel to the plane
3x ⫹ y ⫺ 4z ⫹ 8 ⫽ 0
e. perpendicular to the yz-plane with y-intercept 4 and z-intercept ⫺2
f. perpendicular to the plane x ⫺ 2y ⫹ z ⫽ 6 containing the line
1x, y, z2 ⫽ 12, ⫺1, ⫺12 ⫹ t13, 1, 2 2
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Chapter 8 Test
1. Given the points A11, 2, 42, B12, 0, 32 , and C14, 4, 4 2, determine the
following:
a. the vector and parametric equations for the plane containing these three
points
b. the corresponding Cartesian equation for the plane that contains the three
given points
1
c. if the point with coordinates Q1, ⫺1, ⫺2R lies on the plane
2. The plane p intersects the coordinate axes at 12, 0, 02 , 10, 3, 02 , and 10, 0, 4 2 .
x
y
z
a. Write an equation for this plane, expressing it in the form a ⫹ b ⫹ c ⫽ 1.
b. Determine the coordinates of a normal to this plane.
3. a. Determine a vector equation for the plane containing the origin and the line
!
with equation r ⫽ 12, 1, 3 2 ⫹ t11, 2, 52 , t [ R.
b. Determine the corresponding Cartesian equation for this plane.
4. a. Determine a vector equation for the plane that contains the two lines
!
L 1 : r ⫽ 14, ⫺3, 5 2 ⫹ t12, 0, ⫺3 2, t [ R, and
!
L 2 : r ⫽ 14, ⫺3, 52 ⫹ s15, 1, ⫺1 2 , s [ R.
b. Determine the corresponding Cartesian equation of this plane.
5. a. A line has
x⫺2
y⫺4
⫽ ⫺2 ⫽ z as its symmetric equation. Determine the
4
coordinates of the point where this line intersects the yz-plane.
b. Write a second symmetric equation for this line using the point you found
in 5.
6. a. Determine the angle between p1 and p2 where the two planes are defined
as p1 : x ⫹ y ⫺ z ⫽ 0 and p2 : x ⫺ y ⫹ z ⫽ 0.
b. For the planes p3 : 2x ⫺ y ⫹ kz ⫽ 5 and p4 : kx ⫺ 2y ⫹ 8z ⫽ 9
i. determine a value of k if these planes are parallel
ii. determine a value of k if these planes are perpendicular
c. Explain why the two given equations in 6. b. cannot represent two
identical planes.
7. a. Using a set of coordinate axes in R2, draw a sketch of the line x ⫹ 2y ⫽ 0.
b. Using a set of coordinate axes in R3, draw a sketch of the plane
x ⫹ 2y ⫽ 0.
c. The equation Ax ⫹ By ⫽ 0, A, B ⫽ 0, represents an equation of a plane in
R3. Explain why this plane must always contain the z-axis.
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