Triangular number

Ten pebbles arranged into a triangle

A triangular number is a number that is the sum of all of the natural numbers up to a certain number. For example, 10 is a triangular number because 1 + 2 + 3 + 4 = 10.

A triangular number is calculated by the equation: ${\frac {n(n+1)}{2}}$ 55,5050,500500,50005000,5000050000 are all triangular numbers.