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In triangle ABC, let I be the incenter and Γ be the circumcircle. Extend AI to intersect the circle at L. Show that L is the circumcenter of triangle BIC

 Apr 30, 2022
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This is called the incenter-excenter lemma and is commonly used in olympiad geometry. Its proof is fairly simple. See this proof done by Evan Chen from Taiwan: https://web.evanchen.cc/handouts/Fact5/Fact5.pdf

 May 2, 2022

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