Question: Tick (✓) the correct answer:
How many diagonals are there in an octagon?
(a) 8
(b) 16
(c) 18
(d) 20
Solution:
(d) 20
For a regular $\mathrm{n}$-sided polygon:
Number of diagonals $=: \frac{n(n-3)}{2}$
For an octagon:
$n=8$
$\frac{8(8-3)}{2}=\frac{40}{2}=20$
