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max no of triangles, equilateral triangles inside equilateral

Source: Mathematics Regional Olympiad of Mexico Center Zone 2009 P3

November 10, 2021
combinatorial geometrycombinatorics

Problem Statement

An equilateral triangle ABCABC has sides of length nn, a positive integer. Divide the triangle into equilateral triangles of length 1 1, drawing parallel lines (at distance 1 1) to all sides of the triangle. A path is a continuous path, starting at the triangle with vertex AA and always crossing from one small triangle to another on the side that both triangles share, in such a way that it never passes through a small triangle twice. Find the maximum number of triangles that can be visited.