9. $A$ and $B$ are two points on a circle with center $O$, and $C$ lies outside the circle, on ray $A B$. Given that $A B=24, B C=28, O A=15$, find $O C$.
Solution: 41 . Let $M$ be the midpoint of $A B$; then $\triangle O M B$ is a right triangle with $O B=15, M B=12$, so $O M=9$. Now $\triangle O M C$ is a right triangle with $O M=9, M C=40$, so $O C=41$.