Section 2-8
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.