Maths Olympiad Prep

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Geometry Difficulty 5.2 AIME, harder Find the answer United States

Problem:

A unit square ABCDABCD 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 APB, \angle BPC, \angle CPD, \angle DPA) \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+b100a + b.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

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 APB, \angle BPC, \angle CPD, \angle DPA) > 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 ABCDABCD such that ABX1ABX_1, BCX2BCX_2, CDX3CDX_3, and DAX4DAX_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 APB, \angle BPC, \angle CPD, \angle DPA > 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'. Define C,DC', D' and AA' similarly. It is not hard to see that the circumcircle of square ABCDA'B'C'D' is the desired circle. Now observe that ABD=ABD=60\angle AB'D' = \angle AB'D = 60^{\circ}. Similarly, ADB=60\angle AD'B' = 60^{\circ}, so ABDAB'D' is equilateral. Its height is the distance from AA to BDB'D', 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: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.