Skip to main content
OlympiadHQ

Browse · MathNet

Print

Mongolian 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.

problem
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