Question

Find number of diagonals of octagon?

Answer 1

Draw an octagon, select one vertex and construct each diagonal from this vertex.You will see there are 5 such diagonals. Thus for each of the 8 vertices you can draw 5 diagonals and hence you have constructed 5 8 = 40 diagonals. But you have constructed each diagonal twice, once from each of its ends.

Answer 2

8*5/2
40/2
20 DIAGNOLS