The number of diagonals in a convex nnn-sided polygon is:
n(n−1)n(n-1)n(n−1)
n(n−3)2\frac{n(n-3)}{2}2n(n−3)
n(n−1)2\frac{n(n-1)}{2}2n(n−1)
n2−3nn^2 - 3nn2−3n