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
Problem
Source: Iranian Geometry Olympiad 2021 IGO Elementary p4
Tags: geometry, reflection, trapezoid, concentric circles
qwertyboyfromalotoftime
25.01.2022 18:51
parmenides51 wrote: 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
$\boxed{\text{Solution 1.}}$ Consider point Z on AB in such a way that $AZFD$ is an isosceles trapezoid hence
it is cyclic. Let $O$ be the circumcenter of $\triangle {AFD}$. Since $X$ and $Y$ are the reflections of $E$ and $C$ with respect to $AD$ and $AF$ , and $ZBCF$ is a parallelogram, then $ED = XD = CF = F Y = ZB$
Suppose that $AF$ meets $CY$ at $H.$ Now notice that
$\angle OZB = \angle OZF + \angle FZB = 90^{\circ} -\angle ZDF + \angle BCD = 90^{\circ} - \angle AFD + \angle ADC$
$\angle ODX = \angle ODA + \angle ADX = 90^{\circ} -\angle AFD + \angle ADC$
$\angle OFY = 180^{\circ} - \angle OF A - \angle YFH = 180^{\circ} - (90^{\circ} - \angle ADC) - \angle HFC = 90^{\circ} - \angle AFD + \angle ADC$
These three equality with $ZB = XD = FY$ and $OZ = OD = OF$ gives us that
$ \triangle OZB \cong \triangle ODX \cong \triangle OFY \Longrightarrow OB = OX = OY $
We are done!
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