Question

AB is perpendicular to CD. If the slope of AB is 2/5, what is the slope of CD?​

Answer 1

Given a slope $m$, the slope $m'$ of a line perpendicular to $m$ is the opposite of the inverse of $m$

In other words:

$mm' = -1 \iff m' = -\dfrac{1}{m}$

So, the slope of CD is -5/2.