JAMB 1997 · UME · Q31

A cone with a sector angle of $45^\circ$ is cut out of a circle of radius $r$ cm. Find the base radius of the cone.

Worked solution (try it first)

The arc of the sector becomes the circumference of the cone's base $R$ : $\frac{45}{360} \times 2\pi r = 2\pi R$ .
Divide both sides by $2\pi$ : $R = \frac{45}{360}r = \frac r8$ , option B.