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Team selection tests for JBMO 2018

Saudi Arabia 2018 geometry

Problem

Let be a triangle inscribed in circle with incenter . The lines and intersect again at and . Circumcircle of triangle meets , again at , . Prove that and intersect on .
Solution
Denote as the other intersection of and . We shall prove that is the intersection of and .

By the cyclic quadrilateral, we have Hence , , are collinear, which means belongs to .

Similarly, passes through . These finish our proof.

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleCyclic quadrilateralsAngle chasing