Question 116
Be $k$ and $k'$ two concentric circles with center point $O$. The circle $k'$ is larger than the circle $k$. A straight line through $O$ cutting $k$ in $A$ and $k'$ in $B$, so $O$ between $A$ and $B$ lies. Another line through $O$ cuts $k$ to $E$ and $k'$ to $F$, so $E$ between $O$ and $F$. Show that the radius of $O A E$, the circle with diameter $A B$ and the circle with diameter $E F$ intersect in one point.