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. The first 25 triangular numbers are:1,3,6,10,15,21,28,36,45,55,78,91,105,120, 136,153,171,190,210,231,253,276,300,325,351

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