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Friday, November 14, 2014

Only the twain shall meet, geometry puzzle.

This is a classic problem, in disguise. That problem was open for 20+ years, so if you solve this problem without using the classic result, pat yourself on the back!

You are given a finite set of lines, S in the 2D plane, no two lines of which are parallel and not all are concurrent.

Show that there are two lines (call them p and q) such that no other line in S passes through the intersection point of p and q.

i.e. among all the points of intersections formed by the lines in S, there is a point through which only two lines of S meet.

[Solution]

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