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30th Turkish Mathematical Olympiad

Turkey geometry

Problem

Let be a triangle such that the circle with diameter is tangent to the exterior bisector of . The internal bisector of intersects with the side at point and with the circumcircle of at point . Let be the midpoint of . Prove that the circumcircle of is tangent to .
Solution
Let be the circumcircle of . Let the intersection points of the external bisector of with be , with be and with be . Then, and are parallel since both are perpendicular to , which implies . Looking at the power of with respect to and we obtain therefore , hence . is the perpendicular bisector of the side so and is a parallelogram with . Let be the reflection of with respect to , we will show that two circles are tangent to each other at . is a diameter of hence lies on . and both are perpendicular to hence is a rectangle, so lies on the circumcircle of and it lies on the line passing through the center of both circles, which implies two circles must be tangent to each other at .

Techniques

TangentsRadical axis theoremAngle chasing