Say you have 2 lines in the plane. You are given their slopes.
The angle between 2 lines in the 2D cartesian plane is given by:
tan( a ) = | (m2 – m1) / (1 + m1m2) |
If the lines are perpendicular, then
m1m2 = -1
(1 + m1m2) = 0
then a = 90 degrees and the lines are perpendicular.