| - | ( | a | − | 50 | ) | ( | 2 | a | − | 180 | ) | = | 600 |
| 方程左边 = | - | a | ( | 2 | a | − | 180 | ) | + | 50 | ( | 2 | a | − | 180 | ) |
| = | - | a | × | 2 | a | + | a | × | 180 | + | 50 | ( | 2 | a | − | 180 | ) |
| = | - | a | × | 2 | a | + | 180 | a | + | 50 | × | 2 | a | − | 50 | × | 180 |
| = | - | a | × | 2 | a | + | 180 | a | + | 100 | a | − | 9000 |
| = | - | a | × | 2 | a | + | 280 | a | − | 9000 |
| - | a | × | 2 | a | + | 280 | a | − | 9000 | = | 600 |