Triangular number

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A triangular number is similar to a factorial number, while a factorial is the product of a number and all whole numbers smaller than it, a triangular (often symbolized by placing a capital Delta "△" (because it is triangular) after a number) represents the summing of a number with all whole numbers smaller than it.

Examples

4△ = 4 + 3 + 2 + 1 = 10 (10 pin bowling uses a triangular arrangement.)
5△ = 5 + 4 + 3 + 2 + 1 = 15 (a common billiards arrangement is 15 balls in a triangle.)
17△ = 17 + 16 + ..... + 3 + 2 + 1 = 153

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