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I am looking for the proof of the following proposition:

Proposition. Let $\triangle ABC$ be an arbitrary triangle with circumcenter $O$. Let $A',B',C'$ be a reflection points of the points $A,B,C$ with respect to the incircle of triangle. Consider the three circles $k_1,k_2,k_3$ defined by the points $AOA'$ , $BOB'$ and $COC'$ , respectively. I claim that $k_1$,$k_2$ and $k_3$ meet at a common point $P$.

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The GeoGebra applet that demonstrates this proposition can be found here.

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1 Answer 1

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Let $P$ be the image of $O$ by the inversion with respect to the incircle. Since $IA\cdot IA'=IO\cdot IP=r^2$, we have that $A, A', O, P$ are concyclic, so $P$ is on $k_1$. Similarly $P$ is on the two other circles, which gives the result. Actually we used nothing special about $O$ here, the result still holds if you replace it by any other point.

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    $\begingroup$ ... and incircle to any other circle $\endgroup$ Commented May 26, 2021 at 4:28
  • $\begingroup$ And Circumcenter of those three Circle are also collinear for any arbitrary points and arbitrary Circle. $\endgroup$
    – user423633
    Commented Jun 19, 2022 at 7:34

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