| ( | x | − | 1000 | ) | ( | 200 | + | 1800 | − | x | ) | = | 21000 |
| 方程左边 = | x | ( | 200 | + | 1800 | − | x | ) | − | 1000 | ( | 200 | + | 1800 | − | x | ) |
| = | x | × | 200 | + | x | × | 1800 | − | x | x | − | 1000 | ( | 200 | + | 1800 | − | x | ) |
| = | 2000 | x | − | x | x | − | 1000 | ( | 200 | + | 1800 | − | x | ) |
| = | 2000 | x | − | x | x | − | 1000 | × | 200 | − | 1000 | × | 1800 | + | 1000 | x |
| = | 2000 | x | − | x | x | − | 200000 | − | 1800000 | + | 1000 | x |
| = | 3000 | x | − | x | x | − | 2000000 |
| 3000 | x | − | x | x | − | 2000000 | = | 21000 |