Place one counter on the table-thatis, the first triangular number.

But indeed unity is triangular ; for every triangular number (Triangular numbers are those of which equilateral triangles can be formed in this way:

Now for a little puzzle in triangular numbers.

Every whole number is either triangular , or the sum of two triangular numbers or the sum of three triangular numbers.

The analogous series for triangular numbers is 1, 3, 6, 1, 6, 3, 1, 9, 9.