**Vertical, Horizontal, Parallel, Perpendicular**

**Slope of a Vertical Line**

Undefined

x = 3

Slope of a Horizontal Line

Zero

y = 3

Slope of Parallel Lines

Equal (parallel lines never cross each other)

Because m = 2 for both lines, y = 2x + 2 is parallel to y = 2x + 5

Slope of Perpendicular Lines

Negative Inverse (perpendicular lines cross at a right angle to each other)

Because 2 and – are negative inverses, y = 2x + 2 is perpendicular to y = – x + 4

**
Question
**What is the slope of x = 7

**Answer**

Undefined

**
Question
**What is the slope of y = 8

**Answer**

0

**
Question
**The equations of two lines are y = -9x +4 and y = -9x +7, respectively.

True or False: Are these lines parallel?

**Answer**

True

**Answer Process**

Because the slopes of the two lines are the same, -9, the lines are parallel.

**
Question
**The equations of two lines are y = – x + 4 and y = 8x – 3, respectively.

True or False: Are these lines perpendicular?

**Answer**

True

**Answer Process
**Because the slopes of the two lines are negative inverses, – and 8, the lines are perpendicular.

Practice – Questions

** **1. What is the slope of x = -7?

2. What is the slope of y = -3?

3. The equations of two lines are y = 7x + 5 and y = 7x – 3, respectively.

True or False: Are these lines parallel?

4. The equations of two lines are y = – x + 9 and y = 3x – 6, respectively.

True or False: Are these lines perpendicular?

5. The equations of two lines are y = x + 2 and y = -5x – 4, respectively.

True or False: Are these lines perpendicular?

Practice – Answers

1. Undefined

2. 0

3. True

4. True

5. True