| ( | x | + | 10 | ) | ( | 800 | − | 20 | x | ) | = | 12000 |
| 方程左边 = | x | ( | 800 | − | 20 | x | ) | + | 10 | ( | 800 | − | 20 | x | ) |
| = | x | × | 800 | − | x | × | 20 | x | + | 10 | ( | 800 | − | 20 | x | ) |
| = | 800 | x | − | x | × | 20 | x | + | 10 | × | 800 | − | 10 | × | 20 | x |
| = | 800 | x | − | x | × | 20 | x | + | 8000 | − | 200 | x |
| = | 600 | x | − | x | × | 20 | x | + | 8000 |
| 600 | x | − | x | × | 20 | x | + | 8000 | = | 12000 |