21. In a circle $\omega$ centred at $O, A A^{\prime}$ and $B B^{\prime}$ are diameters perpendicular to each other such that the points $A, B, A^{\prime}, B^{\prime}$ are arranged in an anticlockwise sense in this order. Let $P$ be a point on the minor arc $A^{\prime} B^{\prime}$ such that $A P$ intersects $B B^{\prime}$ at $D$ and $B P$ intersects $A A^{r}$ at $C$. Suppose the area of the quadrilateral $A B C D$ is 100 . Find the radius of $\omega$.