
A triangular number is a number that can be represented by a pattern of dots arranged in an equilateral triangle with the same number of dots on each side.
For example:

The first triangular number is 1, the second is 3, the third is 6, the fourth 10, the fifth 15, and so on.
You can see that each triangle comes from the one before by adding a row of dots on the bottom which has one more dot than the previous bottom row. This means that the
Triangular numbers have lots of interesting properties. For example, the sum of consecutive triangular numbers is a square number. You can see this by arranging the triangular dot patterns representing the

What is more, alternating triangular numbers (1, 6, 15, ...) are also hexagonal numbers (numbers formed from a hexagonal dot pattern) and every even perfect number is a triangular number.
Triangular numbers also come up in real life. For example, a network of
We would like to thank Zoheir Barka who sent us the first draft of this article. We will publish a lovely article by Barka about triangular numbers soon. In the mean time, you can read Barka's article about beautiful patterns in multiplication tables here.