WAEC 2021 · Paper 1 · Q18

The height of an equilateral triangle is $10\sqrt3\text{ cm}$ . Calculate its perimeter.

Worked solution (try it first)

The height of an equilateral triangle of side $s$ is $s\sin60^\circ = \frac{\sqrt3}{2}s$ .
Set $\frac{\sqrt3}{2}s = 10\sqrt3$ and multiply both sides by $\frac{2}{\sqrt3}$ : $s = 20$ cm.
The perimeter is three sides: $3 \times 20 = 60$ cm, option B.