What if you wanted to figure out if two lines in a shape met at right angles? How could you do this? After completing this Concept, you'll be able to use slope to help you determine whether or not lines are perpendicular.
Watch This
CK12 Foundation: Chapter3PerpendicularLinesintheCoordinatePlaneA
Watch the portion of this video that deals with Perpendicular Lines
Khan Academy: Equations of Parallel and Perpendicular Lines
Guidance
Recall that the definition of
perpendicular
is two lines that intersect at a
If we take a closer look at these two lines, we see that the slope of one is 4 and the other is
Example A
Find the slope of the perpendicular lines to the lines below.
a)
b)
c)
We are only concerned with the slope for each of these.
a)
b)
c) Because there is no number in front of
Example B
Find the equation of the line that is perpendicular to
First, the slope is the reciprocal and opposite sign of
5 & = 27 + b \qquad \text{Therefore, the equation of line is} \ y=3x32.\\
32 & = b
Example C
Graph
First, we have to change each equation into slopeintercept form. In other words, we need to solve each equation for
4y & = 3x+8 && 3y = 4x + 15\\
y & = \frac{3}{4}x2 && y = \frac{4}{3}x+5
Now that the lines are in slopeintercept form (also called
Watch this video for help with the Examples above.
CK12 Foundation: Chapter3PerpendicularLinesintheCordinatePlaneB
Vocabulary
Two lines in the coordinate plane with slopes that are opposite signs and reciprocals of each other are
perpendicular
and intersect at a
Guided Practice
1. Determine which of the following pairs of lines are perpendicular.

y=−2x+3 andy=12x+3

y=4x−2 andy=4x+5

y=−x+5 andy=x+1
2. Find the equation of the line that is perpendicular to the line
3. Give an example of a line that is perpendicular to the line
Answers:
1. Two lines are perpendicular if their slopes are opposite reciprocals. The only pairs of lines this is true for is the
first
pair, because
2. The perpendicular line goes through (2, 2), but the slope is
2 & = \frac{1}{2}(2) + b\\
2 & = 1+b\\
1 & =b
The equation is
3. Any line perpendicular to
Interactive Practice
Practice

Determine which of the following pairs of lines are perpendicular.

y=−3x+1 andy=3x−1 
2x−3y=6 and3x+2y=6 
5x+2y=−4 and5x+2y=8 
x−3y=−3 andx+3y=9 
x+y=6 and4x+4y=−16

Determine the equation of the line that is perpendicular to the given line, through the given point.

y=x−1; (−6, 2) 
y=3x+4; (9, −7) 
5x−2y=6; (5, 5) 
y=4; (−1, 3) 
x=−3; (1, 8) 
x−3y=11; (0, 13)
Determine if each pair of lines is perpendicular or not.
For the line and point below, find a perpendicular line, through the given point.