86. How many diagonals can be drawn in a convex $n$-sided polygon?
86. Answer: $C_{n}^{2}-n$. Solution: let's take all pairs of different vertices of the polygon (there are $C_{n}^{2}$ such pairs) and connect the points of each pair with segments. We will get $C_{n}^{2}$ segments. Among them, there will be $n$ sides, and the rest are diagonals, i.e., there will be $C_{n}^{2}-n$ diagonals.