Key Idea: Perpendicular

Two lines are perpendicular if they intersect at a right angle (90°).
Their slopes are negative reciprocals of each other.

General Rule:

If one line has slope $m$, the perpendicular line’s slope is:

m_perpendicular=−(1/m)​

Why this matters for the SAT:

Perpendicular slopes are used in coordinate geometry problems involving right angles, tangents to circles, and equation of a line questions. Remembering to flip the fraction and change the sign ensures accuracy.