1.83: Sector geometry and area

Figure 1 shows a sector $OAB$ of a circle. The circle has centre $O$ and radius 10 cm. The area of the sector is $25\text{ cm}^2$ and angle $AOB=\theta$ radians.

Find

(a) the value of $\theta$, (2)

(b) the length of the arc $AB$. (2)