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n triangles, any 3 of them have common vertex, not 4 of them

Source: 1996 Estonia National Olympiad Final Round grade 11 p5

March 11, 2020
combinatorial geometrycombinatoricsTriangles

Problem Statement

Suppose that nn triangles are given in the plane such that any three of them have a common vertex, but no four of them do. Find the greatest possible nn.