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Tangent in Isosceles Triangle

Source: All Russian Olympiad 2017 10.2 & 11.2

July 5, 2017
tangentgeometryisoscelescircumcircle

Problem Statement

Let ABCABC be an acute angled isosceles triangle with AB=ACAB=AC and circumcentre OO. Lines BOBO and COCO intersect AC,ABAC, AB respectively at B′,C′B', C'. A straight line ll is drawn through C′C' parallel to ACAC. Prove that the line ll is tangent to the circumcircle of △B′OC\triangle B'OC.
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