Maths Olympiad Prep

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Geometry Difficulty 4.9 AIME Find the answer

Alice and Bob are playing in the forest. They have six sticks of length 1,2,3,4,5,61,2,3,4,5,6 inches. Somehow, they have managed to arrange these sticks, such that they form the sides of an equiangular hexagon. Compute the sum of all possible values of the area of this hexagon.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Let the side lengths, in counterclockwise order, be a,b,c,d,e,fa, b, c, d, e, f. Place the hexagon on the coordinate plane with edge aa parallel to the xx-axis and the intersection between edge aa and edge ff at the origin (oriented so that edge bb lies in the first quadrant). If you travel along all six sides of the hexagon starting from the origin, we get that the final xx coordinate must be a+b/2c/2de/2+f/2=0a+b / 2-c / 2-d-e / 2+f / 2=0 by vector addition. Identical arguments tell us that we must also have b+c/2d/2ef/2+a/2=0b+c / 2-d / 2-e-f / 2+a / 2=0 and c+d/2e/2fa/2+b/2=0c+d / 2-e / 2-f-a / 2+b / 2=0. Combining these linear equations tells us that ad=eb=cfa-d=e-b=c-f. This is a necessary and sufficient condition for the side lengths to form an equiangular hexagon. WLOG say that a=1a=1 and b<fb<f (otherwise, you can rotate/reflect it to get it to this case). Thus, we must either have (a,b,c,d,e,f)=(1,5,3,4,2,6)(a, b, c, d, e, f)=(1,5,3,4,2,6) or (1,4,5,2,3,6)(1,4,5,2,3,6). Calculating the areas of these two cases gets either 673/467 \sqrt{3} / 4 or 653/465 \sqrt{3} / 4, for a sum of 33333 \sqrt{3}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.