Triangular number

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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, and 351.

A triangular number is calculated by the equation: .