ISL 2011 G8
Let be an acute triangle with circumcircle . Let be a tangent line to . Let , and be the lines obtained by reflecting in the lines , and , respectively. Show that the circumcircle of the triangle determined by the lines , and is tangent to the circle . To avoid a large case distinction, we will use the notion of oriented angles. Namely, for two lines and , we denote by the angle by which one may rotate anticlockwise to obtain a line parallel to . Thus, all oriented angles are considered modulo . 
Dataset text NuminaMath olympiads. Quoted with source.
Solutions
Official solution Solution 1 by the IMO Problem Selection Committee
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