#1**+5 **

Notice, that from each vertex, 2 diagonals can be drawn, giving us 10 diagonals. But the diagonal drawn from vertex A to vertex D would be the same diagonal drawn fro vertex D to vertex A.

So, we need to divide this by 2 = 5 diagonals.

Here's a pic to demonstrate this.....

BTW....the number of diagonals that can be drawn in any regular polygon is given by....

[n*(n-3)] / 2 .... for n > 3

CPhill
Mar 22, 2015