Question 77
Two circles $\Gamma_1$ and $\Gamma_2$ intersect at points $M$, $N$. A line $l$ is tangent to $\Gamma_1$, $\Gamma_2$ at $A$ and $B$, respectively. The lines passing through $A$ and $B$ and perpendicular to $l$ intersect $MN$ at $C$ and $D$ respectively. Prove that $ABCD$ is a parallelogram.