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jmc

geometry senior

Problem

Four circles of radius 1 are each tangent to two sides of a square and externally tangent to a circle of radius 2, as shown. What is the area of the square?

problem
Solution
Let be the length of a side of the square. Consider an isosceles right triangle with vertices at the centers of the circle of radius 2 and two of the circles of radius 1. This triangle has legs of length 3, so its hypotenuse has length .



The length of a side of the square is 2 more than the length of this hypotenuse, so . Hence the area of the square is
Final answer
22+12\sqrt{2}