[list=1][*]Pause the animation at any point (by clicking the pause button in the bottom left corner).[/*][br][*]Slide the black dot to set the number of sides of the shape you want to investigate.[/*][br][*]Use the check boxes to show/hide all diagonals from a specific vertex.[/*][br][*]Do you see the "[url=https://www.geogebra.org/m/xfhb8dzg]triangle number pattern[/url]"? Try finding some way of multiplying the number of sides by something to get twice the number of diagonals.[/*][/list][br][br][br] [br][br][br]