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