Triangular number

A triangular number is a positive integer of the form n*(n+1)/2 where n is a positive integer. They are called triangular numbers because triangular numbers of things can be arranged in a triangle.

Examples
The first 20 triangular numbers are:

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171, 190, 210

The 100th triangular number is 5050.