How many unique diagonals can be drawn in a five-sided convex polygon?

I looked up what a convex polygon was.

https://www.google.com.au/search?q=what+is+a+convex+polygon&ie=utf-8&oe=utf-8&gws_rd=cr&ei=YhsPVfvGL8jx8gXrUQ

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