Question:
The radius of the circle whose arc of length 15 π cm makes an angle of
(a) 10 cm
(b) 20 cm
(c) $11 \frac{1}{4} \mathrm{~cm}$
(d) $22 \frac{1}{2} \mathrm{~cm}$
Solution:
(b) 20 cm
$\theta=\frac{\text { Arc }}{\text { Radius }}$
$\Rightarrow \frac{3 \pi}{4}=\frac{15 \pi}{\text { Radius }}$
$\Rightarrow$ Radius $=\frac{60}{3}=20 \mathrm{~cm}$