Question 121
Let $AB$ be a diameter of the circle $\mathcal{K}$ circumscribed about the cyclic quadrilateral $ABCD$. The lines $AD$ and $BC$ intersect at the point $E$, and the tangents to the circle $\mathcal{K}$ at the points $C$ and $D$ intersect at the point $F$. Prove that the lines $EF$ and $AB$ are perpendicular.