| ( | 1 | + | 2 | + | 3 | + | 4 | + | 5 | + | 6 | + | 7 | a | ) | ( | 1212 | + | 121 | a | ) | = | 21312 |
| Left side of the equation = | 1 | ( | 1212 | + | 121 | a | ) | + | 2 | ( | 1212 | + | 121 | a | ) | + | 3 | ( | 1212 | + | 121 | a | ) | + | 4 | ( | 1212 | + | 121 | a | ) | + | 5 | ( | 1212 | + | 121 | a | ) | + | 6 | ( | 1212 | + | 121 | a | ) |
| = | 1 | × | 1212 | + | 1 | × | 121 | a | + | 2 | ( | 1212 | + | 121 | a | ) | + | 3 | ( | 1212 | + | 121 | a | ) | + | 4 | ( | 1212 | + | 121 | a | ) | + | 5 |
| = | 1212 | + | 121 | a | + | 2 | ( | 1212 | + | 121 | a | ) | + | 3 | ( | 1212 | + | 121 | a | ) | + | 4 | ( | 1212 | + | 121 | a | ) | + | 5 | ( | 1212 | + | 121 | a | ) | + | 6 |
| = | 1212 | + | 121 | a | + | 2 | × | 1212 | + | 2 | × | 121 | a | + | 3 | ( | 1212 | + | 121 | a | ) | + | 4 | ( | 1212 | + | 121 | a | ) |
| = | 1212 | + | 121 | a | + | 2424 | + | 242 | a | + | 3 | ( | 1212 | + | 121 | a | ) | + | 4 | ( | 1212 | + | 121 | a | ) | + | 5 | ( | 1212 | + | 121 | a | ) |
| = | 3636 | + | 363 | a | + | 3 | ( | 1212 | + | 121 | a | ) | + | 4 | ( | 1212 | + | 121 | a | ) | + | 5 | ( | 1212 | + | 121 | a | ) | + | 6 | ( | 1212 | + | 121 | a | ) | + | 7 |
| = | 3636 | + | 363 | a | + | 3 | × | 1212 | + | 3 | × | 121 | a | + | 4 | ( | 1212 | + | 121 | a | ) | + | 5 | ( | 1212 | + | 121 | a | ) |
| = | 3636 | + | 363 | a | + | 3636 | + | 363 | a | + | 4 | ( | 1212 | + | 121 | a | ) | + | 5 | ( | 1212 | + | 121 | a | ) | + | 6 | ( | 1212 | + | 121 | a | ) |
| = | 7272 | + | 726 | a | + | 4 | ( | 1212 | + | 121 | a | ) | + | 5 | ( | 1212 | + | 121 | a | ) | + | 6 | ( | 1212 | + | 121 | a | ) | + | 7 | a | ( | 1212 | + | 121 | a | ) |
| = | 7272 | + | 726 | a | + | 4 | × | 1212 | + | 4 | × | 121 | a | + | 5 | ( | 1212 | + | 121 | a | ) | + | 6 | ( | 1212 | + | 121 | a | ) |
| = | 7272 | + | 726 | a | + | 4848 | + | 484 | a | + | 5 | ( | 1212 | + | 121 | a | ) | + | 6 | ( | 1212 | + | 121 | a | ) | + | 7 | a |
| = | 12120 | + | 1210 | a | + | 5 | ( | 1212 | + | 121 | a | ) | + | 6 | ( | 1212 | + | 121 | a | ) | + | 7 | a | ( | 1212 | + | 121 | a | ) |
| = | 12120 | + | 1210 | a | + | 5 | × | 1212 | + | 5 | × | 121 | a | + | 6 | ( | 1212 | + | 121 | a | ) | + | 7 | a |
| = | 12120 | + | 1210 | a | + | 6060 | + | 605 | a | + | 6 | ( | 1212 | + | 121 | a | ) | + | 7 | a | ( | 1212 | + | 121 | a | ) |
| = | 18180 | + | 1815 | a | + | 6 | ( | 1212 | + | 121 | a | ) | + | 7 | a | ( | 1212 | + | 121 | a | ) |
| = | 18180 | + | 1815 | a | + | 6 | × | 1212 | + | 6 | × | 121 | a | + | 7 | a | ( | 1212 | + | 121 | a | ) |
| = | 18180 | + | 1815 | a | + | 7272 | + | 726 | a | + | 7 | a | ( | 1212 | + | 121 | a | ) |
| = | 25452 | + | 2541 | a | + | 7 | a | ( | 1212 | + | 121 | a | ) |
| = | 25452 | + | 2541 | a | + | 7 | a | × | 1212 | + | 7 | a | × | 121 | a |
| = | 25452 | + | 2541 | a | + | 8484 | a | + | 847 | a | a |
| = | 25452 | + | 11025 | a | + | 847 | a | a |
| 25452 | + | 11025 | a | + | 847 | a | a | = | 21312 |