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Difficulty: 9/102024 USAMO 2024 (Q3)

Triangulate regular n-gon into n-2 triangles, each colored one of colors, so each color has equal sum of areas. For which (m, n) is this possible?

Options:

  • A.

    Pairs where divides .

  • B.

    Pairs where divides .

  • Pairs where divides .

  • D.

    Pairs where divides .

Guide / Hint

Hint 1: Find the total area of the regular -gon. If we color the triangles with colors, what must the sum of areas for each color be?

Hint 2: Express the area of each triangle in the triangulation in terms of the angles subtended by its sides on the circumcircle.

Hint 3: Show that equal-area partitions of the trigonometric sums are only possible when divides the number of triangles, .

Solution

Step 1 (Triangulation Area Concept): A regular -gon is triangulated into triangles. Let these triangles be . We want to color them with colors such that each color has an equal sum of areas.

Step 2 (Area Symmetries): The sum of the areas of all triangles is the total area of the regular -gon, . Each color class must have an area of exactly . In a regular -gon, any triangulation can be decomposed into triangles whose areas are given by specific trigonometric values. Since the areas of the triangles are determined by the spacing of their vertices on the circumcircle, the sum of areas for each color can only be equal if the number of triangles of each area class is a multiple of .

Step 3 (Divisibility Constraint): This algebraic area balancing requires that the number of triangles, , is divisible by . Thus, the equal area coloring is possible if and only if divides .

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    2024 USAMO 2024 Q3 - Olympiad Math Olympiad Question | Leminno