How to find the number of diagonals of a polygon?

The number of diagonals in the polygon :

A Diagonal is a line segment that connects the opposite corners of a polygon through its vertex.

Number of Diagonals $= \frac{n(n-3)}{2}$,  where n is the number of sides of the polygon.

For example:

Calculate the number of diagonals of a pentagon.

Number of sides of pentagon $= 5$

So, Number of Diagonals $= \frac{n(n-3)}{2}$

                                         $  = \frac{5(5 - 3)}{2}$

                                         $  = \frac{5(2)}{2}$

                                         $  = 5$.