Triangular Numbers

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Triangular numbers can be found by stacking a group of objects in equilateral fashion two objects are placed under one, three objects are placed under the two that are under the one and so on as shown in the illustration.

The fourth triangular number 10 was called the tetraktys by Pythagoras and his followers. They considered it sacred and revered it.

What is so special about the triangular pattern? Can you work out how many objects there are in the eighteenth triangular number?

Posted: December 11, 2007
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Hard 14 The 7 Knights Problem
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