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