Maths Olympiad Prep

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Geometry Difficulty 5.2 AIME, harder Prove it Belarus

Consider a fixed circle Γ\Gamma with three fixed points AA, BB, and CC on it. Also let us fix a real number λ(0,1)\lambda \in (0,1). For a variable point P{A,B,C}P \notin \{A, B, C\} on Γ\Gamma, let MM be the point on the segment CPCP such that CM=λCPCM = \lambda \cdot CP. Let QQ be the second point of intersection of the circumcircles of the triangles AMPAMP and BMCBMC.

Prove that as PP varies, the point QQ lies on a fixed circle.

(IMO-2014 Shortlist, Problem G4)

Solution

3. See IMO-2014 Shortlist, Problem G4.

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