เรขาคณิต
 

Let $ABC$ be a right triangle with $\hat{A}=90^๐$ and let $D$ be a point on side $AC$.
Denote by $E$ the reflection of $A$ across the line $BD$ and $F$ the intersection point of $CE$ with the perpendicular to $BC$ at $D$.Prove that $AF,DE$ and $BC$ are concurrent.