Maths Olympiad Prep

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Geometry Difficulty 5.8 AIME, harder Prove it Croatia

Let RR be a positive integer less than 20172017. Exactly RR vertices of a regular 20172017-gon are red, while the remaining ones are blue. Prove that the number of isosceles triangles having all three vertices of the same colour does not depend on the arrangement of the blue and the red vertices. (Russia 1994)

Solution

First, notice that none of the triangles formed by taking three vertices of the given polygon is equilateral. If there were one, we would have the same number of vertices of the original polygon between any two of the triangle's vertices. This would imply that 33 divides 20142014, which is false.

Let us now prove that each segment (whose endpoints are vertices of the given polygon) belongs to exactly 33 different isosceles triangles.

Figure 1

Consider an arbitrary segment. Its endpoints divide the circumcircle of the polygon into two arcs containing 20152015 vertices. One of the arcs contains an odd number, and the other contains an even number of vertices. Therefore, there exists a unique isosceles triangle with the chosen segment as its base (the third vertex is the midpoint of the arc containing an odd number of vertices). In a similar fashion, we infer that the chosen segment is a base in exactly two isosceles triangles – there are two possible choices of the third segment, both on the longer arc.

Let BB be the number of blue points, so that B+R=2017B + R = 2017. Let db,dr,dsd_b, d_r, d_s, be the number of blue (both endpoints blue), red, and two-colour segments, respectively. Furthermore, let tb,tr,tsb,tsrt_b, t_r, t_{sb}, t_{sr} be the number of isosceles triangles with the vertices coloured all blue, all red, two blue – one red, and two red – one blue, respectively.

From the fact that each segment belongs to three isosceles triangles, we get:
3db=3tb+tsb,3dr=3tr+tsr,3ds=2tsb+2tsr. \begin{aligned} 3d_b &= 3t_b + t_{sb}, \\ 3d_r &= 3t_r + t_{sr}, \\ 3d_s &= 2t_{sb} + 2t_{sr}. \end{aligned}
This implies 3db+3dr32ds=3tb+3tr3d_b + 3d_r - \frac{3}{2}d_s = 3t_b + 3t_r, i.e.
tb+tr=db+dr12ds=12(B(B1)+R(R1)BR), t_b + t_r = d_b + d_r - \frac{1}{2}d_s = \frac{1}{2} \cdot (B(B-1) + R(R-1) - BR),
which depends only on the number of the red and blue points, but not on their arrangement.

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