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Belarus2022

Belarus 2022 geometry

Problem

Two circles and intersect each other at points and . Two lines pass through : one intersects and for the second time at points and respectively and another intersects and for the second time at points and respectively. The line intersects and for the second time at points and respectively such that . Prove that circumcircles of and touch each other. (Palina Chernikava)
Solution
The quadrilaterals and are cyclic, so Since the triangle is isosceles, whence . So the quadrilateral is cyclic. Therefore The last equality is equivalent to the tangency of the circumcircles of the triangles and .

Techniques

Cyclic quadrilateralsTangentsAngle chasing