Maths Olympiad Prep

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, 2025

Geometry Difficulty 6.0 National Olympiad Prove it Hungary

Let OO denote the origin and let γ\gamma be the circle with center (1,0)(1,0) and radius 1 in the Cartesian system of coordinates. Let λ\lambda be a real number from the interval (0,2)(0,2), and let the line x=λx=\lambda intersect the circle γ\gamma at points PP and QQ. The lines OPOP and OQOQ intersect the line x=2λx=2-\lambda at the points PP' and QQ', respectively. Let G\mathcal{G} denote the locus of such points PP' and QQ' as λ\lambda varies over the interval (0,2)(0,2). Prove that there exist points RR and SS different from the origin in the plane such that for every AGA\in \mathcal{G} there exists a point AA' on line OAOA satisfying

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