Consider 9 dots arranged in a three by three square. Connect all these 9 points with four lines without lifting your pen from the paper.

The drawing shows "physical" dots, not "mathematical" dots.

Using 3 straight lines, the solution appears like a big Z, with inclined external lines. Thus, start tracing the first line through the points a, b and c, but tangencing above the point a, across point b, and tangencing below the point c. The second line is the line that pass trought f, e, and d, horizontally (since the first line IS NOT paralel to the second and must cross each other somewhere). The third line is traced the same way the first, trough the points g, h, and i. 

Edited on May 14, 2008, 7:19 pm

Edited on May 14, 2008, 7:21 pm

Posted by pcbouhid on 2008-05-14 13:10:53