triangular number

This is the name given to a number represented by a triangular pattern in dot form.

If we add another row of dots, and count each and every dot, we can find the next number in the sequence.

Here’s the beginning of the sequence:

1 = 1

1 + 2 = 3

1 + 2 + 3 = 6

1 + 2 + 3 + 4 = 10

1 + 2 + 3 + 4 + 5 = 15

…

Therefore, 1, 3, 6, 10, 15 and so on are triangular numbers. The picture opposite shows how the sequence is displayed in dot form.