| ( | x | − | 1000 | ) | ( | 2000 | − | x | ) | ÷ | 50 | = | 4200 |
| 方程左边 = | x | ( | 2000 | − | x | ) | × | 1 50 | − | 1000 | ( | 2000 | − | x | ) | × | 1 50 |
| = | x | ( | 2000 | − | x | ) | × | 1 50 | − | 20 | ( | 2000 | − | x | ) |
| = | x | × | 2000 | × | 1 50 | − | x | x | × | 1 50 | − | 20 | ( | 2000 | − | x | ) |
| = | x | × | 40 | − | x | x | × | 1 50 | − | 20 | ( | 2000 | − | x | ) |
| = | 40 | x | − | x | x | × | 1 50 | − | 20 | × | 2000 | + | 20 | x |
| = | 40 | x | − | x | x | × | 1 50 | − | 40000 | + | 20 | x |
| = | 60 | x | − | x | x | × | 1 50 | − | 40000 |
| 60 | x | − | x | x | × | 1 50 | − | 40000 | = | 4200 |