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smc

geometry senior

Problem

As shown in the figure below, six semicircles lie in the interior of a regular hexagon with side length 2 so that the diameters of the semicircles coincide with the sides of the hexagon. What is the area of the shaded region ---- inside the hexagon but outside all of the semicircles?
problem
(A)
(B)
(C)
(D)
Solution
Let point be a vertex of the regular hexagon, let point be the midpoint of the line connecting point and a neighboring vertex, and let point be the second intersection of the two semicircles that pass through point . Then, , since is the center of the semicircle with radius that lies on, , since is the center of the semicircle with radius that lies on, and , as a regular hexagon has angles of 120, and is half of any angle in this hexagon. Now, using the Law of Sines, , so . Since the angles in a triangle sum to 180, is also 60. Therefore, is an equilateral triangle with side lengths of . Since the area of a regular hexagon can be found with the formula , where is the side length of the hexagon, the area of this hexagon is . Since the area of an equilateral triangle can be found with the formula , where is the side length of the equilateral triangle, the area of an equilateral triangle with side lengths of is . Since the area of a circle can be found with the formula , the area of a sixth of a circle with radius is . In each sixth of the hexagon, there are two equilateral triangles colored white, each with an area of , and one-sixth of a circle with radius colored white, with an area of . The rest of the sixth is colored gray. Therefore, the total area that is colored white in each sixth of the hexagon is , which equals , and the total area colored white is , which equals . Since the area colored gray equals the total area of the hexagon minus the area colored white, the area colored gray is , which equals .
Final answer
D