| ( | a | − | 22 | ) | ( | 1680 | − | 40 | a | ) | = | 3640 |
| 方程左边 = | a | ( | 1680 | − | 40 | a | ) | − | 22 | ( | 1680 | − | 40 | a | ) |
| = | a | × | 1680 | − | a | × | 40 | a | − | 22 | ( | 1680 | − | 40 | a | ) |
| = | 1680 | a | − | a | × | 40 | a | − | 22 | × | 1680 | + | 22 | × | 40 | a |
| = | 1680 | a | − | a | × | 40 | a | − | 36960 | + | 880 | a |
| = | 2560 | a | − | a | × | 40 | a | − | 36960 |
| 2560 | a | − | a | × | 40 | a | − | 36960 | = | 3640 |