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PROBLEM 136

A diagonal in a convex polygon is a line joining two non-adjacent vertices.  Take an n-sided polygon and draw all possible diagonals.  Suppose that no three diagonals ever intersect at a common point.  How many crossings are there in the interior of the polygon?  Below is a sketch of the situation for a pentagon and a hexagon where the number of crossings, c, is five and fifteen, respectively.

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ã2002 Alberto L. Delgado