9.7. Two spheres touch the plane of triangle $A B C$ at points $A$ and $B$ and are located on opposite sides of this plane. The sum of the radii of these spheres is 13, and the distance between their centers is $\sqrt{505}$. The center of a third sphere with radius 8 is located at point $C$, and it touches the first two spheres. Find the radius of the circle circumscribed around triangle $A B C$.
Answer. $2 \sqrt{21} \approx 9.17$