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