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2014 AMC 10A Problem 13

Problem 13 of 25IntermediateGeometry

Equilateral ABC\triangle ABC has side length 1,1, and squares ABDE,ABDE, BCHI,BCHI, CAFGCAFG lie outside the triangle. What is the area of hexagon DEFGHI?DEFGHI?

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Solution

We can find the areas of all the individual pieces and then add them up together. The area of the center equilateral triangle is 1234=34. \dfrac{1^2 \sqrt{3}}{4} = \dfrac{\sqrt{3}}{4}. We have that the areas of all the squares is 312=3. 3 \cdot 1^2 = 3. We also have that EAF=36060290 \angle EAF = 360^{\circ} - 60^{\circ} - 2 \cdot 90^{\circ}=120. = 120^{\circ}. Also, AE=AF=1AE=AF=1 and EAF=120\angle EAF=120^\circ. Dropping the altitude from AA shows that EF=3EF=\sqrt3 and the altitude is 12\frac12, so [EAF]=34[EAF]=\frac{\sqrt3}{4}. The other two outer triangles have the same area. Thus their combined area is 334\frac{3\sqrt3}{4}. The total area is then 34+334+3=3+3. \dfrac{\sqrt{3}}{4} + \dfrac{3\sqrt{3}}{4} + 3 = 3 + \sqrt{3}. Thus, C is the correct answer.

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Concepts: equilateral triangle · square (geometry) · area decomposition

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.