| ( | 10 | x | + | 100 | ) | ( | x | − | 40 | ) | = | 8000 |
| 方程左边 = | 10 | x | ( | x | − | 40 | ) | + | 100 | ( | x | − | 40 | ) |
| = | 10 | x | x | − | 10 | x | × | 40 | + | 100 | ( | x | − | 40 | ) |
| = | 10 | x | x | − | 400 | x | + | 100 | ( | x | − | 40 | ) |
| = | 10 | x | x | − | 400 | x | + | 100 | x | − | 100 | × | 40 |
| = | 10 | x | x | − | 400 | x | + | 100 | x | − | 4000 |
| = | 10 | x | x | − | 300 | x | − | 4000 |
| 10 | x | x | − | 300 | x | − | 4000 | = | 8000 |