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Korean Mathematical Olympiad Final Round

South Korea geometry

Problem

Let be the orthocenter of an acute triangle , and a circle that passes through all of the points , , meet the line segment at the point . Let be the intersecting points of the line and the line segment , and be the circumcenter of the triangle . Show that the center of the circle lies on the circumscribed circle of the triangle .
Solution
Let be the center of the circle , and be the intersecting points of and . Then we have On the other hand, if we let be the midpoint of then , , and are collinear, and it holds that Thus lies on the circumscribed circle of .

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleCyclic quadrilateralsAngle chasing