There are many polygons out there. From rectangles to decagons, and even further, those are all polygons. In fact, a triangle is, too. Diagonal is just a line that isn't top - to - bottom or left -...
In a polygon with n sides, we have n(n-3)/2 diagonals.
In a convex polygon with n sides, you can draw n-3 diagonals from each vertex, but you are counting each one twice you so you need to divide by...