| ( | 2000 | − | 20 | x | ) | ( | x | − | 50 | ) | = | 12000 |
| 方程左边 = | 2000 | ( | x | − | 50 | ) | − | 20 | x | ( | x | − | 50 | ) |
| = | 2000 | x | − | 2000 | × | 50 | − | 20 | x | ( | x | − | 50 | ) |
| = | 2000 | x | − | 100000 | − | 20 | x | ( | x | − | 50 | ) |
| = | 2000 | x | − | 100000 | − | 20 | x | x | + | 20 | x | × | 50 |
| = | 2000 | x | − | 100000 | − | 20 | x | x | + | 1000 | x |
| = | 3000 | x | − | 100000 | − | 20 | x | x |
| 3000 | x | − | 100000 | − | 20 | x | x | = | 12000 |