Perpendicular Slopes Theorem

If two lines are perpendicular then the product of their respective slopes is -1.

This theorem helps us to find the slope of a line, if we know the slope of the line perpendicular to it. 

Line AB is perpendicular to line CD

For example, suppose line AB has a slope of 3 and line CD is perpendicular to it. Suppose we assume slope of CD to be 'm'. By the perpendicular slopes theorem, the product of the slopes of lines AB and CD should be -1. Thus, 

`3 * m = -1`

Solving for 'm', we have

`m = -1/3`

Thus the slope of the line CD perpendicular to line AB is `-1/3`.