$7 \cdot 2$ When two numbers are drawn without replacement from the set $\left\{-3,-\frac{5}{4},-\frac{1}{2}, 0, \frac{1}{3}, 1, \frac{4}{5}, 2\right\}$, find the probability that the two numbers are the slopes of a pair of perpendicular lines.
[Solution] Two numbers represent the slopes of a pair of perpendicular lines, meaning the product of the two numbers is -1. Pairs of numbers that satisfy this condition are: -3 and $\frac{1}{3}$, $-\frac{5}{4}$ and $\frac{4}{5}$, $-\frac{1}{2}$ and 2, for a total of 3 pairs.

Choosing 2 numbers from 8, there are $C_{8}^{2}=\frac{8 \times 7}{2}=28$ ways, so the required probability is $\frac{3}{28}$.