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?
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 , 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 ), we find that the square of the length of the chord is equal to . We can also compute the area of each circle segment, namely . Hence, the desired region has area .
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.