Browse · MathNet
Print42nd Balkan Mathematical Olympiad
geometry
Problem
Let be an acute-angled triangle with , which is inscribed in the circle centered at . Denote by the orthocenter of and by the midpoint of side . Let be the circumcircle of triangle , let the line meet at , and let the line meet at . Prove that the tangent line to at and the tangent line to at meet on the circumcircle of triangle .

Solution
Let be the point such that is a parallelogram and be the center of . Let meet at , and meet at . Furthermore, let be the reflection of across , and be the reflection of across . is a parallelogram, so , which means lies on . We similarly obtain that lies on and lies on . is also a parallelogram, so , i.e. . Since passes through , it is the perpendicular bisector of . From the power of point we deduce , which means is cyclic. It is well-known that is a parallelogram, so and . Therefore, is an isosceles trapezoid, so which means that lies on . We also have , where the last equality holds because . Hence is cyclic. Next we have , so is tangent to . From here we obtain , so is also cyclic. This implies that points all lie on a circle, and so . Now let the tangent line to at and the tangent line to meet at . Then we have and so is cyclic. Therefore, points are concyclic, meaning that lies on , as needed.
Techniques
TangentsCyclic quadrilateralsTriangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleAngle chasingConstructions and loci