Integration — practice question

The diagram depicts sector $AOB$, which is a part of a circle centred at $O$ with radius $6\,\text{cm}$ and with angle $AOB = 0.8$ radians. Point $C$ on $OB$ is chosen so that $AC$ is perpendicular to $OB$. Arc $CD$ is part of a circle with centre $O$, and $D$ lies on $OA$.