Lesson Objectives
• Demonstrate an understanding of slope-intercept form
• Demonstrate an understanding of slope
• Learn how to determine if two lines are parallel
• Learn how to determine if two lines are perpendicular

## How to Determine if two Lines are Parallel Lines or Perpendicular Lines

In this lesson, we will learn how to determine if two lines are parallel lines or perpendicular lines.

### Parallel Lines

Parallel lines are any two lines on a plane that will never intersect. We can determine if two lines are parallel by examining the slope of each. Two non-vertical parallel lines have slopes that are equal. We specified non-vertical here since vertical lines have an undefined slope. Let’s look at an example of parallel lines. Suppose we encounter the following two equations:
-2x + y = 5
4x - 2y = 6
If we solve each for y:
y = 2x + 5
y = 2x - 3
In each case, we can see that the slope is the same (2). The y-intercepts are different (0,5) and (0,-3). Since each line has the same slope or steepness, they will never touch each other. Let's look at a graph for further illustration: We can see from our graph that these two lines will never intersect.

### Perpendicular Lines

Perpendicular Lines are lines that intersect at a 90° angle. Two non-vertical perpendicular lines have slopes whose product is -1. Let's look at an example of perpendicular lines. Suppose we encounter the following two equations:
3x + 2y = 4
2x - 3y = 3
If we solve each for y:
y = -3/2 x + 2
y = 2/3 x - 1
The slope of the first equation is -3/2, while the slope of the second equation is 2/3. If we multiply the two slopes together, we get a product of (-1): $$-\frac{3}{2}\cdot \frac{2}{3}$$ $$\require{cancel}-\frac{\cancel{3}}{\cancel{2}}\cdot \frac{\cancel{2}}{\cancel{3}}=-1$$ Since our two slopes multiply together to give us a product of (-1), we know our lines are perpendicular. Let's look at a graph for further illustration: We can see from our graph that these two lines intersect at a 90° angle. Let's look at a few examples.
Example 1: Determine if each pair of lines are parallel, perpendicular, or neither.
6x - 5y = 12
12x - 10y = -15
Solve each for y:
y = 6/5 x - 12/5
y = 6/5 x + 3/2
We can see that each slope of each line is 6/5. This tells us we have parallel lines.
Example 2: Determine if each pair of lines are parallel, perpendicular, or neither.
7x - 2y = 5
2x + 7y = 84
If we solve each for y:
y = 7/2 x - 5/2
y = -2/7 x + 12
Our two slopes (7/2) and (-2/7) are not equal. Therefore, we know that we don't have parallel lines. We can multiply the slopes together to determine if we have perpendicular lines. We are looking for a product of (-1): $$\frac{7}{2}\cdot -\frac{2}{7}$$ $$\frac{\cancel{7}}{\cancel{2}}\cdot -\frac{\cancel{2}}{\cancel{7}}=-1$$ We can see that the product of the slopes is (-1). This tells us we have perpendicular lines.
Example 3: Determine if each pair of lines are parallel, perpendicular, or neither.
-8x - 3y = 12
-5x + y = 20
If we solve each for y:
y = -8/3 x - 4
y = 5x + 20
Our two slopes (-8/3) and 5 are not equal. We can multiply the slopes together to determine if we have perpendicular lines. We are looking for a product of (-1): $$-\frac{8}{3}\cdot 5 ≠ -1$$ We can see the product of the slopes is not (-1), therefore, these lines are not perpendicular. We can say these two lines are not parallel lines and they are not perpendicular lines either.

#### Skills Check:

Example #1

Determine if parallel, perpendicular, or neither. $$5x + y=-15$$ $$y=-5x + 4$$

A
Parallel
B
Perpendicular
C
Neither

Example #2

Determine if parallel, perpendicular, or neither. $$2x - y=3$$ $$y=-\frac{1}{2}x - 4$$

A
Parallel
B
Perpendicular
C
Neither

Example #3

Determine if parallel, perpendicular, or neither. $$7x + 3y=-9$$ $$y=\frac{2}{7}x - 4$$

A
Parallel
B
Perpendicular
C
Neither         