Maths Olympiad Prep

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Geometry Difficulty 4.9 AIME Find the answer

Call a polygon normal if it can be inscribed in a unit circle. How many non-congruent normal polygons are there such that the square of each side length is a positive integer?

A number or a short expression. Spacing and $ signs are ignored.

Solution

The side lengths of the polygon can only be from the set {1,2,3,2}\{1, \sqrt{2}, \sqrt{3}, 2\}, which take up 60,90,120,18060^{\circ}, 90^{\circ}, 120^{\circ}, 180^{\circ} of the circle respectively. By working modulo 60 degrees we see that 2\sqrt{2} must be used an even number of times. We now proceed to casework on the longest side of the polygon. Case 1: If the longest side has length 2, then the remaining sides must contribute the remaining 180 degrees. There are 3 possibilities: (1,1,1,2),(1,3,2),(2,2,2)(1,1,1,2),(1, \sqrt{3}, 2),(\sqrt{2}, \sqrt{2}, 2). Case 2: If the longest side has length 3\sqrt{3}, then it takes up either 120120^{\circ} or 240240^{\circ} of the circle. In the former case we have 6 possibilities: (1,1,1,1,3),(1,2,2,3),(2,1,2,3),(1,1,3,3)(1,1,1,1, \sqrt{3}),(1, \sqrt{2}, \sqrt{2}, \sqrt{3}),(\sqrt{2}, 1, \sqrt{2}, \sqrt{3}),(1,1, \sqrt{3}, \sqrt{3}), (1,3,1,3),(3,3,3)(1, \sqrt{3}, 1, \sqrt{3}),(\sqrt{3}, \sqrt{3}, \sqrt{3}). In the latter case there is only 1 possibility: (1,1,3)(1,1, \sqrt{3}). Case 3: If the longest side has length 2\sqrt{2}, then it shows up either twice or four times. In the former case we have 2 possibilities: (1,1,1,2,2),(1,1,2,1,2)(1,1,1, \sqrt{2}, \sqrt{2}),(1,1, \sqrt{2}, 1, \sqrt{2}). In the latter case there is only 1 possibility: (2,2,2,2)(\sqrt{2}, \sqrt{2}, \sqrt{2}, \sqrt{2}). Case 4: If all sides have length 1, then there is 1 possibility: (1,1,1,1,1,1)(1,1,1,1,1,1). Adding up all cases, we have 3+6+1+2+1+1=143+6+1+2+1+1=14 polygons.

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