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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.
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