In a 2d diamond there are 2 diagonals.
how many triangles are formed when any parallelogram and it diagonals are drawn
An octagon has 16 diagonals.
0.5*(82-24) = 20 diagonals
Four sided quadrilaterals have two diagonals.
If all of the diagonals are drawn from a vertex of an octagon, how many triangles are formed
You can draw 9 diagonals inside a hexagon.