Answer
$\frac{s^{2}}{2\theta}$
Work Step by Step
$s=r\theta\implies r=\frac{s}{\theta}$
Then,
$A=\frac{1}{2}r^{2}\theta=\frac{1}{2}\times(\frac{s}{\theta})^{2}\times\theta$
$=\frac{1}{2}\times \frac{s^{2}}{\theta^{2}}\times\theta=\frac{s^{2}}{2\theta}$
