Geometry · Orthocentre · Circumcircle · Inscribed angle · Law of sines
A circle is drawn through two vertices of a triangle and through the orthocentre of that triangle. Prove that this circle has the same radius as the circle circumscribed about the triangle.
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Serbian Regional Competition (Okruzno takmicenje) 2006, high school grade I, category A, problem 1. Organized by the Mathematical Society of Serbia (DMS). Source