Linear Equations negative one. Non Vertical Lines

Linear Equations
Non Vertical Lines
Ex: 1, 2, 3, 4, & 5
Parallel and Perpendicular Lines
Ex: 6, 7 & 8
General Form
Ex: 9
Proof of the product of the slopes of two perpendicular lines is
negative one.
Using Similar Triangle
Using Pythagorean Theorem
Equation of a non Vertical Line
Let’s P0 =(0,b) be the point of intersection of a non vertical line and the yaxis ( b is usually called the y-intercept).
For any two (arbitrary) points P1(x1, y1) & P2 = (x2 , y2) we define
P2
the slope of P1P2 as
If P0, P1 , P2 are on the same line, then
mP0 P1  mP1P2
Proof: Since the blue and the red right triangles are similar
as shown in the figure, so the ratios of the corresponding
sides are in proportion, therefore:
AP1 BP2

AP0 BP1

y1  y0 y2  y1

x1  x0 x2  x1
 mP0 P1  mP1P2
y2-y1
P1
x2-x1 B
y1-y0
P0
x1-x0
A
b
This means that the line has an unique slope and we will call it m.
If P=(x,y) is an arbitrary point on the line besides P0 then :
Any point (x,y) satisfying the equation y = mx+b
lies on the (non vertical) line and vice-versa.
Slope-intercept
equation
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More of non Vertical Lines
Let  be a line with slope m, and P1(x1 , y1) a given point
Then for any arbitrary point P(x,y) on , the slope m is given by:
Multiplying both side by x - x1 , we get the equation of 
y – y1 = m (x - x1 )
Point-slope equation
Example1: Find m and b for the line 10x – 5y = 4
Solving for y
5y = 10x – 4  y = 2x – 4/5
Comparing with y = mx + b we have
m = 2 & b = – 4/5
Example 2: Find the equation of line passing though (5,-2) & (-3,-4).
Using slope formula
Point-slope equation using
Working on the equation
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Example 3: Find x if (x, -5/4) is a point on the line 8x-20y = 7.
Since (x, -5/4) satisfies the equation 8x-20(-5/4) = 7  8x + 25 = 7  x = –
9/4
Example 4: Graph of line y = 2x – 4
We need to find two points on the line.
Let’s fill the blanks in the following table
x
y
0
-4
2
0
2
If x = 0 ; y =2(0) – 4 = – 4
So, A = (0, -4) is on the line.
If y=0 ; 0 = 2x – 4  x=2
So, B = (2 , 0) is on the line.
-4
Once we plot the points, we can draw the line.
Example 5: Find slope of the line as shown in the
figure.
Pick two points on the line, whose coordinates can
read accurately, such as P1= (0,3) & P2 = (4,1).
Using P1 & P2 , we get:
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1
1
Parallel & Perpendicular Lines
Example 6: Given the line 3x+5y =30,
find the area of OAB in the figure.
From figure A = (10,0) & B = (0, 6)
Since OAB is a right triangle
So, Area = ½( OA )(OB) =½ (10)(6) = 30.
Parallel & Perpendicular Lines
Let L1 & L2 be two non vertical lines:
L1 // L 2  m1 = m2
L1  L2  m1 m2 = -1
Click here to see the proof
Example 7: Specify the following lines that are //, or , or neither.
L1:
L2:
L3:
L4:
y = -2x + 5
4x +2y = 7
5x + 10 y = 3
x – 2y = 8




m1 = - 2
m2 =-2
m3 = -1/2
m4 = 1/2
L1 // L2
L1  L4, & L2  L4
But L1&L3; L2&L3; and L3&L4 are
neither.
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Example 8: Find an equation of the line passing through (15, 4) and is
a) // to the line 5y = 3x + 15
b)  to the line 5y = 3x + 15
Solving for y , we get:
y = (3/5)x +3  m = 3/5
a) Two parallel lines have slope the same, so m = 3/5
The equation should have the form
y = (3/5)x + b
Since the line passing through (15,4)
4 =(3/5)15 + b  b = -5
So the required parallel line is y = (3/5)x – 5
b) Perpendicular line has slope –1/m, which is equal to – 1/(3/5) = – 5/3
The equation should have the form y = –(5/3)x + b
Since the line passing through (15,4) 4 = –(5/3)15 + b  b = 29
So the perpendicular line is y = –(5/3)x + 29
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General Equation of a Line
The linear equation Ax + By = C (with A & B are not simultaneously zero) is
called the general equation of a line (in the Cartesian Plane), because:
If B  0, then solving for y we get y = -Ax /B + C/B , that is, y = mx + b with
m = -A/B & b= C/B. So the graph is a line.
If B = 0, then A  0. So Ax = C, then x = C/A and the graph is a line parallel to
the Y axis (called a vertical line ). Note that for vertical lines, their slopes are not
defined.
So the graph of Ax + By = C (with A & B not simultaneously zero) is a line.
Equivalently, every point (x,y) not on the line Ax+By=C satisfying Ax+ By  C, is
either in Ax+ By >C or in Ax+By < C. The line divides the plane into two regions
and it can be proved that all points in the plane satisfying one of the inequalities
are located in the same region. This idea will be crucial to solve (by graphing )
systems of linear inequalities.
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Linear Equations in General or Standard From
Example 9: Find an equation of the line with slope – 2/3 and y
intercept 2. Write the final answer in standard form.
Solution:
Use slope-intercept formula:
y = mx + b with m = -2/3 & b = 2.
We have:
y = (-2/3)x + 2
Multiply both sides by 3: 3y = -2x +6
Move – 2x to left side and we have
the required equation in standard
form: 2x + 3y = 6
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The product of the slopes of two
perpendicular lines is negative one.
1. Prove using similar triangles.
Let L1 & L2 be perpendicular lines. For simplicity, we shift
the point of intersection of lines in figure A to the origin.
We draw the perpendicular line at x=1 and determine
the red and the blue right triangles (as shown in figure).
Blue and red triangles are similar.

So the ratio of the corresponding sides are in
proportion: We form the ratio of the opposite side
to the adjacent side with respect to the angle 
And we have


y1
1

So y1 | y2 |  1
1 | y2 |
The slope of line L1 is m1 
y1
 y1
1
y2
m

 y2
The slope of line L2 is
2
1
y1 x
1

 m1 m2  1  m1m2  1
For any x: m1 = =
x y2
m2
Therefore the product of the slopes is negative one.
Continue with second Proof
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y1

1

|y2|
1
The product of the slopes of two perpendicular
lines is negative one.
2. Prove using Pythagorean Theorem.
For simplicity, we shift the point of intersection of
two perpendicular lines L1 & L2 to the origin.
Using the Pythagorean Theorem:
On red right triangle:
On blue right triangle:
 OA 
OB
2
2
 12  m12  1  m12
But
AB  m1  m2
o
 12  m22  1  m22
 AB    OA   OB
2
On right triangle OAB:
A
2
2
. Thus
Therefore the product of the slopes is negative one.
B
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