WAEC 2025 · Paper 1 · Q29✱✱

A $120^\circ$ sector of a circle of radius $21\text{ cm}$ is bent to form a cone. What is the base radius of the cone?

Worked solution (try it first)

The arc of the sector becomes the circumference of the base: $\frac{120}{360} \times 2\pi \times 21 = 14\pi$ .
So $2\pi r = 14\pi$ .
Divide both sides by $2\pi$ : $r = 7$ cm, option D.