Respuesta
$s = r \cdot \theta$, where $r$ is the radius and $\theta$ is measured in radians.
Explicación
If angle is given in degrees, convert via $\theta_{\text{rad}} = \theta_{\text{deg}} \times \frac{\pi}{180^\circ}$. Sector area is $A = \frac{1}{2}r^2\theta$.