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smc

geometry senior

Problem

Four regular hexagons surround a square with side length 1, each one sharing an edge with the square, as shown in the figure below. The area of the resulting 12-sided outer nonconvex polygon can be written as , where , , and are integers and is not divisible by the square of any prime. What is ?
problem
(A)
(B)
(C)
(D)
Solution
Refer to the diagram above. Let the origin be at the center of the square, be the intersection of the top and right hexagons, be the intersection of the top and left hexagons, and and be the top points in the diagram. By symmetry, lies on the line . The equation of line is (due to it being one of the sides of the top hexagon). Thus, we can solve for the coordinates of by finding the intersection of the two lines: This means that we can find the length , which is equal to . We will next find the area of trapezoid . The lengths of the bases are and , and the height is equal to the -coordinate of minus the -coordinate of . The height of the hexagon is and the bottom of the hexagon lies on the line . Thus, the -coordinate of is , and the height is . We can now find the area of the trapezoid: The total area of the figure is the area of a square with side length plus four times the area of this trapezoid: Our answer is .
Final answer
B