Question

Tick (✓) the correct answer:
How many diagonals are there in an octagon?
(a) 8
(b) 16
(c) 18
(d) 20

Answer 1

(d) 20

​For a regular n-sided polygon:
Number of diagonals =: $\frac{n\left(n-3\right)}{2}$
For an octagon:

$$\begin{gathered} n=8 \\ \frac{8\left(8-3\right)}{2}=\frac{40}{2}=20 \end{gathered}$$