Question

The number of diagonals that can be drawn by joining the vertices of an octagon is
(a) 20
(b) 28
(c) 8
(d) 16

Answer 1

(a) 20

An octagon has 8 vertices.
The number of diagonals of a polygon is given by $\frac{n\left(n-3\right)}{2}$.
∴ Number of diagonals of an octagon = $\frac{8\left(8-3\right)}{2}=20$