Question 96
in a circle $\mathcal{C}$ centered $O$, we draw a string $[CD]$. Let $\Gamma$ the circle of diameter $[CD]$ and $O'$ its center. the mediator of $[CD]$ cut the circle $\mathcal{C}$ in two points $A$ and $B$ such as $A$ is outside $\Gamma$. Notes $T$ and $T'$ the contact points of the tangents at $\Gamma$ via $A$. Let $F$ the middle of $[TT']$. To show that $O'$ is the middle of $[BF]$.