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Geometry Difficulty 6.4 National Olympiad Prove it Japan

Suppose a regular 2010-gon is given. How many combinations of 3 distinct vertices AA, BB, CC of this polygon are there for which all the inner angles of the triangle ABCABC have integer-valued degrees (i.e., integral multiples of 11^\circ)? Regard 2 triples as representing the same combination if one is a permutation of the other.

Solution

272020 \boxed{272020}
Let aa be the number of the sides of the 2010-gon lying in between the vertices BB and CC and on the opposite side from the vertex AA, and bb be the number of the sides lying in between the vertices CC and AA and on the opposite side from the vertex BB. Let Γ\Gamma be the circum-circle of this regular 2010-gon, and let OO be its center. Then, we have
CAB=12COB=12×360×a2010=(6a67) \angle CAB = \frac{1}{2} \angle COB = \frac{1}{2} \times 360^\circ \times \frac{a}{2010} = \left( \frac{6a}{67} \right)^\circ
In the same way, we get ABC=(6b67)\angle ABC = \left(\frac{6b}{67}\right)^\circ. When these two angles take the integral values, then the BCA\angle BCA also takes an integral value. Hence the condition of the problem is satisfied if and only if both 6a6a and 6b6b are divisible by 6767, which is the same as the statement that both aa and bb are multiples of 6767 (since 6767 is a prime).
From 201067=30\frac{2010}{67} = 30, it also follows that this is equivalent to the statement that AA, BB, CC are the vertices of a regular 30-gon inscribed in the circle Γ\Gamma. There are 201030=67\frac{2010}{30} = 67 regular 30-gons whose vertices are chosen from the given regular 2010-gon and for each of these regular 30-gons there are (303)\binom{30}{3} ways of choosing 3 vertices. Therefore, there are 67×(303)=27202067 \times \binom{30}{3} = 272020 triangles satisfying the condition of the problem.

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