Problem

Source: Iranian Geometry Olympiad 2021 IGO Elementary p4

Tags: geometry, reflection, trapezoid, concentric circles



In isosceles trapezoid $ABCD$ ($AB \parallel CD$) points $E$ and $F$ lie on the segment $CD$ in such a way that $D, E, F$ and $C$ are in that order and $DE = CF$. Let $X$ and $Y$ be the reflection of $E$ and $C$ with respect to $AD$ and $AF$. Prove that circumcircles of triangles $ADF$ and $BXY$ are concentric. Proposed by Iman Maghsoudi - Iran