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Team Selection Test for IMO

Turkey geometry

Problem

Let be a triangle with . Interior and exterior angle bisectors at vertex intersect at and respectively. A variable point lies on the ray such that is closer to than . Lines and intersect at point and lines and intersect at point . Prove that all lines when a variable point changes intersect at a unique point. (Ali Adali).
Solution
Let . Menelaus' theorem applied to and collinear points yields: Menelaus' theorem applied to and collinear points yields: Side by side division gives: Finally, Menelaus' theorem applied to and collinear points yields: Therefore, (the last equality follows from ). Thus, .

Techniques

Menelaus' theoremTrianglesConstructions and loci