| ( | 190 | − | 5 | a | ) | ( | a | − | 12 | ) | = | 720 |
| 方程左边 = | 190 | ( | a | − | 12 | ) | − | 5 | a | ( | a | − | 12 | ) |
| = | 190 | a | − | 190 | × | 12 | − | 5 | a | ( | a | − | 12 | ) |
| = | 190 | a | − | 2280 | − | 5 | a | ( | a | − | 12 | ) |
| = | 190 | a | − | 2280 | − | 5 | a | a | + | 5 | a | × | 12 |
| = | 190 | a | − | 2280 | − | 5 | a | a | + | 60 | a |
| = | 250 | a | − | 2280 | − | 5 | a | a |
| 250 | a | − | 2280 | − | 5 | a | a | = | 720 |