I1.1 The given figure shows a circle of radius $18 \mathrm{~cm}$, centre $O$.
If $\angle A O B=\frac{\pi}{3}$ and the length of arc $A P B$ is $a \pi \mathrm{cm}$, find $a$.
$\begin{array}{l}a \pi=18 \times \frac{\pi}{3} \\ a=6\end{array}$