Formula of regular polygon diagonal

The n-sided polygon has $\frac{ n (n-3) }{ 2 }$ diagonals.

Sides Diagonals
3 0
4 2
5 5
6 9
7 14
8 20
9 27
10 35
11 44
12 54

\[ d = \frac{ n (n-3) }{ 2 } \]

d: Number of diagonals
n: Number of sides

Regular polygon

Formula