[Similar auxiliary triangles] Trapezoids (other). $\quad]$

In trapezoid $A B C D$, the lateral side $A B$ is perpendicular to the bases $A D$ and $B C$, the diagonals of the trapezoid intersect at point $E, F$ - the foot of the perpendicular dropped from point $E$ to side $A B$. It is known that $\angle D F E=\alpha$. Find $\angle C F E$.
Right triangles $B C F$ and $A D F$ are similar, since $B F: A F=C E: A E=D C: A D$. Therefore, $\angle C F E=\angle B C F=\angle A D F=\angle D F E=\alpha$.

## Answer

$\alpha$.