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PrintMongolian Mathematical Olympiad
Mongolia geometry
Problem
Let be an acute triangle, where the altitudes and are drawn. Let be the point on segment such that , and let be the point on segment such that . Denote as the midpoint of segment . The circumcircle of triangle intersects line again at point and line again at point . Prove that the lines , , and are concurrent.

Solution
Let . Since is the midpoint of , it follows that . Since , or is an isosceles trapezoid. Applying the sine theorem on Applying the sine theorem on , From (1) and (2) Since is an isosceles trapezoid, , and points and are bases, so or the lines , , intersect at one point.
Techniques
Triangle trigonometryConcurrency and CollinearityCyclic quadrilateralsAngle chasing