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 |