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HMMT 2020 G10

  • Geometry
  • Answer

Let Γ\Gamma be a circle of radius 1 centered at OO. A circle Ω\Omega is said to be friendly if there exist distinct circles ω1,ω2,…,ω2020\omega_{1}, \omega_{2}, \ldots, \omega_{2020}, such that for all 1≤i≤2020,ωi1 \leq i \leq 2020, \omega_{i} is tangent to Γ,Ω\Gamma, \Omega, and ωi+1\omega_{i+1}. (Here, ω2021=ω1\omega_{2021}=\omega_{1}.) For each point PP in the plane, let f(P)f(P) denote the sum of the areas of all friendly circles centered at PP. If AA and BB are points such that OA=12O A=\frac{1}{2} and OB=13O B=\frac{1}{3}, determine f(A)−f(B)f(A)-f(B).

Dataset text NuminaMath olympiads. Quoted with source.

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\frac{1000}{9} \pi

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