MathDB
Convex polygon with nine vertices

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January 11, 2007

Problem Statement

The figure shows a (convex) polygon with nine vertices. The six diagonals which have been drawn dissect the polygon into the seven triangles: P0P1P3P_{0}P_{1}P_{3}, P0P3P6P_{0}P_{3}P_{6}, P0P6P7P_{0}P_{6}P_{7}, P0P7P8P_{0}P_{7}P_{8}, P1P2P3P_{1}P_{2}P_{3}, P3P4P6P_{3}P_{4}P_{6}, P4P5P6P_{4}P_{5}P_{6}. In how many ways can these triangles be labeled with the names 1\triangle_{1}, 2\triangle_{2}, 3\triangle_{3}, 4\triangle_{4}, 5\triangle_{5}, 6\triangle_{6}, 7\triangle_{7} so that PiP_{i} is a vertex of triangle i\triangle_{i} for i=1,2,3,4,5,6,7i = 1, 2, 3, 4, 5, 6, 7? Justify your answer. 6740