Question 136812
Given {{{L[1]}}}: {{{y=m[1]x+b[1]}}} and {{{L[2]}}}: {{{y=m[2]x+b[2]}}}, {{{L[1]}}} is perpendicular to {{{L[2]}}} if and only if {{{m[1]=-1/m[2]}}}, which is to say that the slopes are the negative reciprocal of each other.


Given the angle measure between two intersecting lines:  The two lines are perpendicular if and only if the angle measure is either 90 degrees or {{{pi/2}}} radians.


That's 57 words, give or take.