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Perpendicular Lines
See It in Action
Two examples of the idea done right—and one common look-alike that is not right.
EXAMPLE
EXAMPLE
NON-EXAMPLE
Definition
Perpendicular lines are lines that intersect at a right angle, which means they meet to form a 90-degree angle.
In Coordinate Geometry:
If two non-vertical lines are perpendicular, their slopes are negative reciprocals of each other.
So if line A has a slope of m, line B is perpendicular to it if its slope is -1/m.
Example 1:
Line A: y = 3x + 2
Perpendicular Line B: y = -1/3x + 5
Why? The slope of 3 becomes -1/3 → negative reciprocal.
Example 2:
Line A: y = -2/5x + 7
Perpendicular Line B: y = 5/2x - 1
Why? The slope of -2/5 becomes 5/2.
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