26 The diagram shows a sector $O A B$ of a circle, centre $O$ and radius $8 \mathrm{~cm}$, in which $\angle A O B=120^{\circ}$. Another circle of radius $r \mathrm{~cm}$ is to be drawn through the points $O$, $A$ and $B$. Find the value of $r$.
26. Answer: 8
Consider the line $O P$ such that $\triangle B O P$ is equilateral. Similarly, $\triangle A O P$ is equilateral. Hence $P A=P O=P B \Rightarrow P$ is the centre of the circle that passes through the points $O, A$ and $B$. Clearly, $r=O P=8 \mathrm{~cm}$.