# Chapter 3: Parallel and Perpendicular Lines

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Chapter 3: Parallel and Perpendicular Lines
3-6 Lines in the Coordinate Plane

Objectives: Student will be able:
1.To write the equations of lines in slope intercept form 2.To graph lines given their equation HW/CW: pgs 169 #’s 2 – 16, Even

Slope-Intercept Form y = mx + b m is the slope b is the y-intercept You need m and b to write the equation!

Write the equation of the line.
What is the y-intercept? b = -2 What is m? m = 3/2 y = 3/2x -2 x y

Write the equation of the line.
b = ? b = 1 m = ? m = -1/2 y = -1/2x + 1 x y

Write the equation of the line.
b = ? b = 4 m = ? m = 0 y = 4 x y

Find the slope and y-int. of each equation.
1) y = -4x + 3 m = -4 b = 3 2) y = 5 - 1/2x m = -1/2 b = 5 3) 8x + y = 3/4 m = -8 b = 3/4 4) 4x - 2y = 10 m = 2 b = -5

Type 1: Write the equation given the slope and y-intercept
1) m = -3, b = 1 y = -3x + 1 2) m = -2/5, b = -4 y = -2/5x - 4

x- and y-intercepts The x-intercept is the point where a line crosses the x-axis. The general form of the x-intercept is (x, 0). The y-coordinate will always be zero. The y-intercept is the point where a line crosses the y-axis. The general form of the y-intercept is (0, y). The x-coordinate will always be zero.

Finding the x-intercept
For the equation 2x + y = 6, we know that y must equal 0. What must x equal? Plug in 0 for y and simplify. 2x + 0 = 6 2x = 6 x = 3 So (3, 0) is the x-intercept of the line.

Finding the y-intercept
For the equation 2x + y = 6, we know that x must equal 0. What must y equal? Plug in 0 for x and simplify. 2(0) + y = 6 0+y = 6 y = 6 So (0, 6) is the y-intercept of the line.

To summarize…. To find the x-intercept, plug in 0 for y.
To find the y-intercept, plug in 0 for x.

Find the x- and y-intercepts of x = 4y - 5
x-intercept: Plug in y = 0 x = 4y-5 x = 4(0) - 5 x = 0 - 5 x = -5 (-5, 0) is the x-int. y-intercept: Plug in x = 0 x = 4y-5 0 = 4y - 5 5 = 4y 5/4 = y (0, 5/4) is the y-int.

Find the x- and y-intercepts of y = -3x – 1
x-intercept Plug in y=0 y = -3x - 1 0 = -3x - 1 1 = -3x -1/3 = x (-1/3, 0) is the x-int. y-intercept Plug in x=0 y = -3(0) - 1 y = 0 - 1 y = -1 (0, -1) is the y-int.

Find the x- and y-intercepts of 6x - 3y =-18
x-intercept Plug in y=0 6x-3y=-18 6x-3(0) = -18 6x - 0 = -18 6x = -18 x = -3 (-3, 0) is the x-int. y-intercept Plug in x=0 6x-3y=-18 6(0)-3y = -18 0 - 3y = -18 -3y = -18 y = 6 (0, 6) is the y-int.

Find the x- and y-intercepts of x=3
x-intercept Plug in y=0 There is no y. Why? X=3 is a vertical line so x always equals 3. (3, 0) is the x-int. y-intercept A vertical line never crosses the y-axis. There is no y-int. x y

Find the x- and y-intercepts of y=-2
x-intercept Plug in y=0 Y cannot = 0 because y=-2. y=-2 is a horizontal line so it never crosses the x-axis. There is no x-int. y-intercept Y=-2 is a horizontal line so y always equals -2. (0,-2) is the y-int. x y

Graphing Equations Example: Graph the equation -5x + y = 2
Solve for y first. -5x+y=2 Add 5x to both sides. y=5x+2 The equation y=5x+2 is in slope-intercept form, y=mx+b. The y-intercept is 2 and the slope is 5. Graph the line on the coordinate plane.

1. Graph y=5x+2 x y

Example :Graph the equation 4x - 3y = 12.
Graphing Equations Example :Graph the equation 4x - 3y = 12. Solve for y first. 4x-3y=12 Subtract 4x from both sides. -3y = -4x Divide by -3. y = -4/-3 x+ 12/-3. Simplify. y = 4/3 x – 4 The equation y=4/3 x - 4 is in slope-intercept form, y=mx+b. The y-intercept is -4 and the slope is 4/3. Graph the line on the coordinate plane.

2. Graph y=4/3 x - 4 x y