1. Lesson 2: Slope of Parallel and Perpendicular Lines
Linear Functions
Lesson 2: Slope of Parallel and Perpendicular Lines

2. Today’s Objectives
Demonstrate an understanding of slope with respect to: rise and run; rate of change; and line segments and lines, including:
Apply a rule for determining whether two lines are parallel or perpendicular
Solve problems involving slope

3. Vocabulary Slope Rise Run The measure of a lines steepness
(vertical change/horizontal change)
Rise
The vertical change of a line
Run
The horizontal change of a line

4. Parallel Lines
Parallel lines are straight lines that do not meet; they remain the same distance apart.
In order for two lines to stay parallel, they must have the same rise and the same run over the same space.
In other words, they must have the same slope.
For example, if the slope of one parallel line is 2/3, the slope of the other parallel line must also be 2/3.

5. Perpendicular Lines
Perpendicular lines are lines that meet at a 90 degree angle.
In order to be perpendicular, the slope of one line must be the negative reciprocal of the slope of the other line.
For example, if the slope of one perpendicular line is 2/3, the slope of the other perpendicular line must be: -3/2.

6. Example The points A(2,-6) and B(4,-3) lie on a line.
A) Find the slope of a line parallel to this line
m = [-3-(-6)]/4-2
m = 3/2
B) Find the slope of a line perpendicular to this line
-2/3
Solution:
Use the given points in the slope formula to find the original slope, then, take the negative reciprocal to find the slope of the perpendicular line.

7. Classifying Shapes
The ability to classify lines as either perpendicular or parallel is helpful when classifying shapes such as triangles, parallelograms, and rectangles.
Example:
Is triangle ABC a right triangle?
A(5,8) B(0,5) C(3,0) A B C

8. Example
Solution:
Measure the slopes of the three lines connecting the points. If two of the slopes are negative reciprocals of each other then we know this is a right triangle.
Side AB
8-5/5-0 = 3/5
Side BC
0-5/3-0 = -5/3
Sides AB and BC are perpendicular,
So this is a right triangle A B C

9. Identifying a Line Perpendicular to a Given Line
Example:
A) Determine the slope of a line that is perpendicular to the line through E(2,3) and F(-4,-1)
B) Determine the coordinates of G so that line EG is perpendicular to line EF.
Solution to A):
Determine the slope of EF:
Slope of EF = −1−3 −4−2 = −4 −6 = 2 3
 The slope of a line perpendicular to EF is the negative reciprocal of 2/3, which is -3/2

10. Identifying a Line Perpendicular to a Given Line
Solution to B):
To find the point G, draw line EF on a graph. The slope of line EG must be -3/2 as it is perpendicular to line EF. From point E, we can count out a rise of -3, and a run of 2 to find a point, G (4, 0). Draw a line through EG on the graph to see that it is perpendicular E G F

11. Your turn
On the following graph, determine the slope of a line that is perpendicular to the line through G(-2,3) and H(1,-2). Determine the coordinates of J so that line GJ is perpendicular to line GH.
J (3,6) H G
J (-7,0)

12. Homework Pg
Even numbered questions

13. Wall Quiz!