Number of Dominoes in a Double-n Set

The formula for calculating how many dominoes are in a double-n set is:

x = (n + 1)(n + 2) / 2

double-n set (n + 1)(n + 2) / 2 x dominoes
0 (0 + 1)(0 + 2) / 2 1
1 (1 + 1)(1 + 2) / 2 3
2 (2 + 1)(2 + 2) / 2 6
3 (3 + 1)(3 + 2) / 2 10
4 (4 + 1)(4 + 2) / 2 15
5 (5 + 1)(5 + 2) / 2 21
6 (6 + 1)(6 + 2) / 2 28
7 (7 + 1)(7 + 2) / 2 36
8 (8 + 1)(8 + 2) / 2 45
9 (9 + 1)(9+ 2) / 2 55
10 (10+ 1)(10 + 2) / 2 66
11 (11+ 1)(11 + 2) / 2 78
12 (12+ 1)(12 + 2) / 2 91

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