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PrintIranian Mathematical Olympiad
Iran geometry
Problem
Consider a triangle with incircle that is respectively tangent to sides , and at , and . Points , are inside of so that , and , . Prove that , and are collinear.
Solution
It's obvious that --- So , also Hence , and are collinear.
Techniques
Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleTangentsAngle chasing