| ( | 550 | − | 5 | x | ) | ( | x | − | 50 | ) | = | 4000 |
| 方程左边 = | 550 | ( | x | − | 50 | ) | − | 5 | x | ( | x | − | 50 | ) |
| = | 550 | x | − | 550 | × | 50 | − | 5 | x | ( | x | − | 50 | ) |
| = | 550 | x | − | 27500 | − | 5 | x | ( | x | − | 50 | ) |
| = | 550 | x | − | 27500 | − | 5 | x | x | + | 5 | x | × | 50 |
| = | 550 | x | − | 27500 | − | 5 | x | x | + | 250 | x |
| = | 800 | x | − | 27500 | − | 5 | x | x |
| 800 | x | − | 27500 | − | 5 | x | x | = | 4000 |