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?
Solution
The side lengths of the polygon can only be from the set , which take up of the circle respectively. By working modulo 60 degrees we see that 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: . Case 2: If the longest side has length , then it takes up either or of the circle. In the former case we have 6 possibilities: , . In the latter case there is only 1 possibility: . Case 3: If the longest side has length , then it shows up either twice or four times. In the former case we have 2 possibilities: . In the latter case there is only 1 possibility: . Case 4: If all sides have length 1, then there is 1 possibility: . Adding up all cases, we have polygons.