| ( | 500 | − | 10 | x | ) | ( | 10 | + | x | ) | = | 8000 |
| 方程左边 = | 500 | ( | 10 | + | x | ) | − | 10 | x | ( | 10 | + | x | ) |
| = | 500 | × | 10 | + | 500 | x | − | 10 | x | ( | 10 | + | x | ) |
| = | 5000 | + | 500 | x | − | 10 | x | ( | 10 | + | x | ) |
| = | 5000 | + | 500 | x | − | 10 | x | × | 10 | − | 10 | x | x |
| = | 5000 | + | 500 | x | − | 100 | x | − | 10 | x | x |
| = | 5000 | + | 400 | x | − | 10 | x | x |
| 5000 | + | 400 | x | − | 10 | x | x | = | 8000 |