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Ukraine geometry
Problem
Non-intersecting circles and of radii are inscribed into angle with
, , and the radius of is smaller than that of . Let and be the points of intersection of with and respectively. Let also and be the points of intersection of with and of with respectively. Prove that and the circumcenters of triangles and are collinear.

, , and the radius of is smaller than that of . Let and be the points of intersection of with and respectively. Let also and be the points of intersection of with and of with respectively. Prove that and the circumcenters of triangles and are collinear.
Solution
Проведемо дотичну до описаного кола трикутника . Тоді , а тому . Отже, . З цього випливає, що пряма дотикається до описаного кола трикутника . Оскільки описані кола трикутників і в точці мають спільну дотичну, то точка лежить на лінії центрів цих кіл.
Techniques
TangentsAngle chasingTriangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circle