olymmath

ISL 2011 G8

    Let ABCA B C be an acute triangle with circumcircle ω\omega. Let tt be a tangent line to ω\omega. Let ta,tbt_{a}, t_{b}, and tct_{c} be the lines obtained by reflecting tt in the lines BC,CAB C, C A, and ABA B, respectively. Show that the circumcircle of the triangle determined by the lines ta,tbt_{a}, t_{b}, and tct_{c} is tangent to the circle ω\omega. To avoid a large case distinction, we will use the notion of oriented angles. Namely, for two lines ℓ\ell and mm, we denote by ∠(ℓ,m)\angle(\ell, m) the angle by which one may rotate ℓ\ell anticlockwise to obtain a line parallel to mm. Thus, all oriented angles are considered modulo 180∘180^{\circ}.

    Dataset text NuminaMath olympiads. Quoted with source.

    Solutions

    Official solution Solution 1 by the IMO Problem Selection Committee

    The terms of the source let olymmath link to this solution but not copy it. Read the solution (IMO official site).

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