Let the incircle of acute triangle be , the circumcircle be , and let the midpoint of side be . Let the incircle be tangent to , at points , respectively, and let line meet the circumcircle at points , . Take a point on the circumcircle of such that is perpendicular to . Prove that line , circle , and circle meet at a single point.
Solution
Let meet at point , and let be tangent to at point . Because , we have , that is, are concyclic. Let meet again at point , and let meet again at point . Then, since , we know , that is, are collinear.

Suppose meets again at point , and let the radical axis of and be . Considering , , , by the radical center theorem we know are concurrent; considering next , by the radical center theorem we know are concurrent, so are concurrent at point , and are concyclic.
Therefore
that is, are collinear, which completes the proof.
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