Ex. 25. The extensions of the altitudes of triangle $ABC$, dropped from vertices $A$ and $B$, intersect the circumcircle of the triangle at points $A'$ and $B'$, respectively. Find $AB$, if $A'B'=12$, and the radius of the circumcircle is 10.
Ex. 25. $AB=6 \sqrt{10}$ or $2 \sqrt{10}$.