IMO Shortlist 2011 problem G6
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Avg: 8,0 Let be a triangle with and let be the midpoint of . The angle bisector of intersects the circle through and at the point inside the triangle . The line intersects the circle through and in two points and . The lines and meet at a point , and the lines and meet at a point . Show that is the incentre of triangle .
Proposed by Jan Vonk, Belgium and Hojoo Lee, South Korea
Proposed by Jan Vonk, Belgium and Hojoo Lee, South Korea
Izvor: Međunarodna matematička olimpijada, shortlist 2011