Maths Olympiad Prep

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Geometry Difficulty 5.1 AIME, harder Find the answer

Four unit circles are centered at the vertices of a unit square, one circle at each vertex. What is the area of the region common to all four circles?

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

Solution

The desired region consists of a small square and four "circle segments," i.e. regions of a circle bounded by a chord and an arc. The side of this small square is just the chord of a unit circle that cuts off an angle of 3030^{\circ}, and the circle segments are bounded by that chord and the circle. Using the law of cosines (in an isosceles triangle with unit leg length and vertex angle 3030^{\circ}), we find that the square of the length of the chord is equal to 232-\sqrt{3}. We can also compute the area of each circle segment, namely π1212(1)(1)sin30=π1214\frac{\pi}{12}-\frac{1}{2}(1)(1) \sin 30^{\circ}=\frac{\pi}{12}-\frac{1}{4}. Hence, the desired region has area 23+4(π1214)=π3+132-\sqrt{3}+4\left(\frac{\pi}{12}-\frac{1}{4}\right)=\frac{\pi}{3}+1-\sqrt{3}.

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