Answer
$(a)$ $s=r\theta$
$(b)$ $A=\frac{1}{2}r^2\theta$
Work Step by Step
$(a)$ The length $s$ of an arc that subtends a central angle of $\theta$ radians is simply $r\theta$, as we learned from the chapter $6$
$s=r\theta$
$(b)$ The are $A$ of a sector with central angle of $\theta$ radians is $\frac{1}{2}r^2\theta$.
Also mentioned in the chapter.
$A=\frac{1}{2}r^2\theta$
