Question

The number of diagonals of a 18-sided polygon is_______

Answer 1

Answer: 135

Given, number of sides $(n)=18$.
Number of diagonals of an $n$-sided polygon
$=\frac{n(n-3)}{2},\text{ where }n=18$
$=\frac{18(18-3)}{2}=\frac{18(15)}{2}=135$