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Team selection tests for BMO 2018

Saudi Arabia 2018 geometry

Problem

Let be a triangle with as midpoints of the segments respectively. Suppose that is the intersection of angle bisectors of , and is the intersection of angle bisectors of , . Denote () as the circle of center and tangent to at , as the circle of center and tangent to at .

1. Prove that is parallel to .

2. Prove that the radical axis of two circles (), () bisects the segment .

problem
Solution
1) Note that then by angle chasing, we have . Denote then is the incenter of triangle . Hence, is the angle bisector of , thus . Denote then Similarly, . Thus . Since then . Therefore, which implies that .

2) Suppose that cuts (), () at respectively. We have Hence . Denote as midpoint of then which implies that lies on the radical axis of .

Techniques

TrianglesTriangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleTriangle trigonometryTangentsRadical axis theoremAngle chasing