Let be the circumcircle of a triangle . A circle passing through points and meets the sides and at and , respectively. The lines and meet again at and , respectively. The tangent lines of at and meet the line at and , respectively. Prove that the lines and meet at .
Solution
Let meet at . Since and , we have . Thus
and hence is tangent to the circumcircle of . Therefore .
Let be the circumcircle of . If , then, since , the tangent lines of and at should coincide, that is is tangent to from inside. Let . If lies in the same side of the line as , then we have
because . That is, the quadrilateral is cyclic, and hence is on the intersection of with .
Otherwise,
Therefore the quadrilateral is cyclic, and hence again is on the intersection of with .
Similarly, if meets at , we either have , in which case is tangent to from inside, or . In the latter case, is on the intersection of with . In either case, we have .
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