1.19: Tangents and the area outside a sector

In Figure 2, $\displaystyle AB$ and $\displaystyle AC$ are tangents to a circle with centre $\displaystyle O$ and radius $\displaystyle r\,\mathrm{cm}$.

The points $\displaystyle B$ and $\displaystyle C$ lie on the circle so that $\displaystyle OBC$ is a sector of this circle and

Given that the area of the shaded region is $\displaystyle 10\,\mathrm{cm}^2$, find, to 3 significant figures, the value of $\displaystyle r$.