Section 12-2
Parallel and Perpendicular Lines
Parallel lines are lines with the same slope. They never intersect.
Example:
The two lines
y = 3x + 8
y = 3x – 4
are parallel, since they both have slope 3.

Perpendicular lines are lines that intersect at right angles. If you multiply the slopes of two perpendicular lines, you get –1.
Example:
The lines

are perpendicular, since

Exception: A horizontal line and a vertical line are perpendicular, even though the product of their slopes
is undefined.