Let $ABCDE$ be a pentagon inscribe in a circle $(O)$. Let $ BE \cap AD = T$. Suppose the parallel line with $CD$ which passes through $T$ which cut $AB,CE$ at $X,Y$. If $\omega$ be the circumcircle of triangle $AXY$ then prove that $\omega$ is tangent to $(O)$.
Problem
Source: Iran Third Round 2013 - Geometry Exam - Problem 1
Tags: geometry, circumcircle, geometric transformation, homothety, geometry unsolved
05.09.2013 23:19
EDIT: There was originally a typo in this question. Now that it has been edited, I can go ahead and write a correct proof. First, note that $\angle AEC=\angle ADC=\angle YTD$ by a combination of cyclic quads and parallel lines, so quadrilateral $AEYT$ must be cyclic. Therefore $\angle AYX=\angle AEB=\angle APB$ and $BP\parallel XY$. This means that triangles $\triangle AXY$ and $\triangle ABP$ are homothetic with center of homothety $A$, implying that their circumcircles are homothetic. Therefore, since the circumcircle of $\triangle ABP$ is simply $(O)$, circles $(O)$ and $\omega$ are internally tangent, as desired. $\blacksquare$ EDIT2: Oops, I forgot the case where $X$ and $Y$ are outside $(O)$. I'm sure the solution is very similar, although several changes need to be made as a result of configuration issues.
05.09.2013 23:35
Thanks , typo edited.
07.09.2013 18:04
As above, $A,T,E,Y$ are concyclic, hence $\angle AYT=\angle AET$, let $\alpha$ being the common angle value. Seeing that $\angle XAO=\angle BAO=90^\circ-\alpha$ we infer that the circumcenter of $\Delta AXY$ belongs to $(AO$, so being done. Best regards, sunken rock
17.09.2013 18:05
@sunken : rock very very beautiful!!!!!! can any body solve with menelaus i tried but i didn't success
22.11.2013 04:49
Let $AY$ meet the circumcircle of $ABCDE$ at $F$.clearly $ATYE$ is cyclic but $\angle AEB=\angle AFB$, hence we get $\angle AYX=\angle AFB$,now if we draw a tangent $MA$ to $\omega $ at $A$ then we get $\angle MAX=\angle AYX=\angle AFB$,so $MA$ is also tangent to the circumcircle of $ABCDE$,hence the result.
23.11.2013 05:20
djmathman wrote: EDIT: There was originally a typo in this question. Now that it has been edited, I can go ahead and write a correct proof. First, note that $\angle AEC=\angle ADC=\angle YTD$ by a combination of cyclic quads and parallel lines, so quadrilateral $AEYT$ must be cyclic. Therefore $\angle AYX=\angle AEB=\angle APB$ and $BP\parallel XY$. This means that triangles $\triangle AXY$ and $\triangle ABP$ are homothetic with center of homothety $A$, implying that their circumcircles are homothetic. Therefore, since the circumcircle of $\triangle ABP$ is simply $(O)$, circles $(O)$ and $\omega$ are internally tangent, as desired. $\blacksquare$ EDIT2: Oops, I forgot the case where $X$ and $Y$ are outside $(O)$. I'm sure the solution is very similar, although several changes need to be made as a result of configuration issues. What's P?
23.11.2013 17:01
Oops. I forgot to mention that $P$ is the second intersection point of $\omega$ and $AY$.
07.04.2014 09:33
From $A$,draw a tangent $AL$ to circle $(O)$. $\angle{TAE}=\angle{DAE}=\angle{DCE}=\angle{CYX}=\angle{CYT} \Rightarrow T,A,Y,E$ concyclic.So $\angle{LAB}=\angle{AEB}=\angle{AET}=\angle{AYT}=\angle{AYX}$ or $AL$ is also tangent to $\omega$.The result follows..
07.01.2020 19:11