Suppose a regular 2010-gon is given. How many combinations of 3 distinct vertices , , of this polygon are there for which all the inner angles of the triangle have integer-valued degrees (i.e., integral multiples of )? Regard 2 triples as representing the same combination if one is a permutation of the other.
Solution
Let be the number of the sides of the 2010-gon lying in between the vertices and and on the opposite side from the vertex , and be the number of the sides lying in between the vertices and and on the opposite side from the vertex . Let be the circum-circle of this regular 2010-gon, and let be its center. Then, we have
In the same way, we get . When these two angles take the integral values, then the also takes an integral value. Hence the condition of the problem is satisfied if and only if both and are divisible by , which is the same as the statement that both and are multiples of (since is a prime).
From , it also follows that this is equivalent to the statement that , , are the vertices of a regular 30-gon inscribed in the circle . There are regular 30-gons whose vertices are chosen from the given regular 2010-gon and for each of these regular 30-gons there are ways of choosing 3 vertices. Therefore, there are triangles satisfying the condition of the problem.