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Baltic Way 2023 Shortlist

Baltic Way 2023 geometry

Problem

In an acute triangle with , the angle bisector of intersects side and at the points and , respectively. Let points and be the feet of perpendiculars from to sides and , respectively. The tangent of at the point and the tangent of at the point intersect at the point . Suppose that lines and intersect at the point . Show that passes through the midpoint of segment .
Solution
Therefore, quadrilateral is cyclic as well. Combining this with being cyclic, gives us that is cyclic. Moreover, note that This means that is tangent to . Also note that .

Now let be the midpoint of , then , since is an isosceles triangle. Combining this with , we have that is a cyclic quadrilateral, as desired.

Techniques

TangentsCyclic quadrilateralsAngle chasing