Let there be an acute scalene triangle $ABC$ with circumcenter $O$. Denote $D,E$ be the reflection of $O$ with respect to $AB,AC$, respectively. The circumcircle of $ADE$ meets $AB$, $AC$, the circumcircle of $ABC$ at points $K,L,M$, respectively, and they are all distinct from $A$. Prove that the lines $BC,KL,AM$ are concurrent.
Problem
Source: KJMO 2018 p5
Tags: geometry, geometric transformation, reflection, circumcircle
26.07.2019 12:12
Nice problem! With an easy angle chasing you get $\angle LKA=\angle LCB$ then $BLCK$ is cyclic. Inversion centered at $A$ with radius $\sqrt{AB\cdot AK}$. Line $BC$ is the image of $(ADE)$ and line $KL$ is the image of $ABC$ then the intersection $P=BC\cap KL$ is the image of $M=(ADE)\cap (ADE)$, which implies $A,P,M$ are collinear and we are done.$\Box$
22.09.2019 19:35
Any elementary solution??
22.09.2019 19:53
ayan_mathematics_king wrote: Any elementary solution?? From #2 we get $CLBK$ is cyclic. Hence, $AM$ is the radical axis of $\odot(ABC)$ and $\odot(ADE)$, $KL$ is the radical axis of $\odot(BKCL)$ and $\odot(ADE)$ and $BC$ is the radical axis of $\odot(BKCL)$ and $\odot(ABC)$. Hence, $AM,BC,KL$ concur.
20.09.2020 12:14
I solved like post no.4 Post no.2 is more creative YEAH!!!
17.02.2022 14:50
$\angle BKL = \angle AKL = \angle ADL = \angle AOD = \angle ACB = \angle BCL$ so $BKCL$ is cyclic. Let $KL$ meet $BC$ at $M$. $BM.MC = KM.ML$ so $M$ has same power w.r.t both $ADE$ and $ABC$ so $M$ lies on radical axis of them so $M$ lies on $AM$.
25.03.2024 11:46
Why DOL is collinear?