Let $ABCD$ be a trapezoid with $AD \parallel BC$ and $\angle BCD < \angle ABC < 90^\circ$. Let $E$ be the intersection point of the diagonals $AC$ and $BD$. The circumcircle $\omega$ of $\triangle BEC$ intersects the segment $CD$ at $X$. The lines $AX$ and $BC$ intersect at $Y$, while the lines $BX$ and $AD$ intersect at $Z$. Prove that the line $EZ$ is tangent to $\omega$ iff the line $BE$ is tangent to the circumcircle of $\triangle BXY$.
We have $\angle DBX = 180^{\circ} - \angle BYX = \angle CYX = \angle DAX$, so $DABX$ is a cyclic quadrilateral. Therefore, $\overline{ZA} \cdot \overline{ZD} = \overline{ZB} \cdot \overline{ZX}$, so $Z$ lies on the radical axis of $(AED)$ and $(BEC)$. It is well-known that $(AED)$ and $(BEC)$ are tangent to each other, so $Z$ must lie on the internal common tangent of those two circles, hence $ZE$ is tangent to $\omega$.