Using your Head is Permitted

August 2007 riddle

This month's riddle was related to me by Guy Bensky.

Let S be a set consisting of a finite number of points on the plane that satisfies the following criterion: any straight line that passes through more than one point of S, passes through at least 3 points of S.

Prove that all points in S are arranged on a straight line.

List of solvers:

Oded Margalit (1 August 23:00)

Elegant solutions can be submitted to the puzzlemaster at Names of solvers will be posted on this page. Notify if you don't want your name to be mentioned.

The solution will be published at the end of the month.


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