Maths Olympiad Prep

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Geometry Difficulty 5.0 AIME, harder Prove it United States

Problem:

Determine the number of convex polygons all of whose sides are the square roots of positive integers which can be inscribed in a unit circle. Polygons that are rotations or reflections of each other are considered the same.

Solution

Solution:

Any chord in a unit circle has length at most 22 (if it is a diameter), so all side lengths must be 11, 2\sqrt{2}, 3\sqrt{3}, or 22. We can do casework based on the number of sides:

- Triangles: The possible combinations of side lengths are (2,2,2)(\sqrt{2}, \sqrt{2}, 2), (1,1,3)(1,1, \sqrt{3}), (1,3,2)(1, \sqrt{3}, 2), and (3,3,3)(\sqrt{3}, \sqrt{3}, \sqrt{3}). This is 44 possibilities.

- Quadrilaterals: The possible ordered combinations of side lengths are (1,1,1,2)(1,1,1,2), (1,3,2,2)(1, \sqrt{3}, \sqrt{2}, \sqrt{2}), (1,3,1,3)(1, \sqrt{3}, 1, \sqrt{3}), (1,3,3,1)(1, \sqrt{3}, \sqrt{3}, 1), (2,2,2,2)(\sqrt{2}, \sqrt{2}, \sqrt{2}, \sqrt{2}), and (1,2,3,2)(1, \sqrt{2}, \sqrt{3}, \sqrt{2}). This is 66 possibilities.

- Pentagons: The possible combinations of side lengths are (1,1,1,1,1,3)(1,1,1,1,1, \sqrt{3}), (1,1,1,2,2)(1,1,1, \sqrt{2}, \sqrt{2}), and (1,1,2,1,2)(1,1, \sqrt{2}, 1, \sqrt{2}). This is 33 possibilities.

- Hexagons: The only option is that all sides are 11, so there is 11 possibility.

Putting these together, there are 4+6+3+1=144+6+3+1=14 polygons total.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.