Triangular Number

A triangular number counts the objects that can be arranged in an equilateral triangle, formed by adding the natural numbers up to some value n, so the nth triangular number equals n(n+1)/2. The sequence begins 1, 3, 6, 10, 15 and grows quadratically. Triangular numbers appear in combinatorics as the count of ways to choose two items from n+1, in figurate-number geometry, and in the identity linking them to square numbers, since consecutive triangular numbers sum to a perfect square. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/