Find the number of diagonals that can be drawn in an n-side polygon

1. Sep 25, 2005

fork

Find the number of diagonals that can be drawn in an n-side polygon.

I don't know how to do that.

2. Sep 25, 2005

Tide

Set up a recursion for the number of diagonals in an n-gon: $D_{n+1} = D_n + n - 2$.

3. Sep 25, 2005

fork

I don't understand how can I set up
$D_{n+1} = D_n + n - 2$.

4. Sep 25, 2005

HallsofIvy

How many diagonals can be drawn from 1 vertex? If you multiply that by the number of vertices you will get the wrong answer! Do you see why? How can you fix it?