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MathNet 2026 — Part 10 Question 14

Math Olympiad Vault · MathNet 2026 — Part 10

Question 14

Question illustration
As shown in the diagram above, the vertices of a regular decagon are colored alternately black and white. We would like to draw colored line segments between the points in such a way that


a. Every black point is connected to every white point by a line segment.


b. No two line segments of the same color intersect, except at their endpoints.

Determine, with proof, the minimum number of colors needed.