| ( | - | 10 | x | + | 1000 | ) | ( | x | − | 30 | ) | = | 10000 |
| 方程左边 = | - | 10 | x | ( | x | − | 30 | ) | + | 1000 | ( | x | − | 30 | ) |
| = | - | 10 | x | x | + | 10 | x | × | 30 | + | 1000 | ( | x | − | 30 | ) |
| = | - | 10 | x | x | + | 300 | x | + | 1000 | ( | x | − | 30 | ) |
| = | - | 10 | x | x | + | 300 | x | + | 1000 | x | − | 1000 | × | 30 |
| = | - | 10 | x | x | + | 300 | x | + | 1000 | x | − | 30000 |
| = | - | 10 | x | x | + | 1300 | x | − | 30000 |
| - | 10 | x | x | + | 1300 | x | − | 30000 | = | 10000 |