Question: a square has how many diagonals?
Answer: a square (or any quadrilateral) has 4(4−3)/2 = 4×1/2 = 2 diagonals. an octagon has 8(8−3)/2 = 8×5/2 = 20 diagonals. a triangle has 3(3−3)/2 = 3×0/2 = 0 diagonals.