Maths Olympiad Prep

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Geometry Difficulty 5.3 AIME, harder Find the answer

A unit square ABCDA B C D and a circle Γ\Gamma have the following property: if PP is a point in the plane not contained in the interior of Γ\Gamma, then min(APB,BPC,CPD,DPA)60\min (\angle A P B, \angle B P C, \angle C P D, \angle D P A) \leq 60^{\circ}. The minimum possible area of Γ\Gamma can be expressed as aπb\frac{a \pi}{b} for relatively prime positive integers aa and bb. Compute 100a+b100 a+b.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Note that the condition for Γ\Gamma in the problem is equivalent to the following condition: if min(APB,BPC,CPD,DPA)>60\min (\angle A P B, \angle B P C, \angle C P D, \angle D P A)>60^{\circ}, then PP is contained in the interior of Γ\Gamma. Let X1,X2,X3X_{1}, X_{2}, X_{3}, and X4X_{4} be the four points in ABCDA B C D such that ABX1,BCX2,CDX3A B X_{1}, B C X_{2}, C D X_{3}, and DAX4D A X_{4} are all equilateral triangles. Now, let Ω1,Ω2,Ω3\Omega_{1}, \Omega_{2}, \Omega_{3}, and Ω4\Omega_{4} be the respective circumcircles of these triangles, and let the centers of these circles be O1,O2,O3O_{1}, O_{2}, O_{3}, and O4O_{4}. Note that the set of points PP such that APB,BPC,CPD,DPA>60\angle A P B, \angle B P C, \angle C P D, \angle D P A>60^{\circ} is the intersection of Ω1,Ω2,Ω3\Omega_{1}, \Omega_{2}, \Omega_{3}, and Ω4\Omega_{4}. We want to find the area of the minimum circle containing this intersection. Let Γ1\Gamma_{1} and Γ2\Gamma_{2} intersect at BB and BB^{\prime}. Define C,DC^{\prime}, D^{\prime} and AA^{\prime} similarly. It is not hard to see that the circumcircle of square ABCDA^{\prime} B^{\prime} C^{\prime} D^{\prime} is the desired circle. Now observe that ABD=ABD=60\angle A B^{\prime} D^{\prime}=\angle A B^{\prime} D=60^{\circ}. Similarly, ADB=60\angle A D^{\prime} B^{\prime}=60^{\circ}, so ABDA B^{\prime} D^{\prime} is equilateral. Its height is the distance from AA to BDB^{\prime} D^{\prime}, which is 12\frac{1}{\sqrt{2}}, so its side length is 63\frac{\sqrt{6}}{3}. This is also the diameter of the desired circle, so its area is π469=π6\frac{\pi}{4} \cdot \frac{6}{9}=\frac{\pi}{6}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.