| ( | 480 | − | x | ) | ( | 160 | − | 480 | + | x | ) | = | 20000 |
| 方程左边 = | 480 | ( | 160 | − | 480 | + | x | ) | − | x | ( | 160 | − | 480 | + | x | ) |
| = | 480 | × | 160 | − | 480 | × | 480 | + | 480 | x | − | x | ( | 160 | − | 480 | + | x | ) |
| = | 76800 | − | 230400 | + | 480 | x | − | x | ( | 160 | − | 480 | + | x | ) |
| = | - | 153600 | + | 480 | x | − | x | ( | 160 | − | 480 | + | x | ) |
| = | - | 153600 | + | 480 | x | − | x | × | 160 | + | x | × | 480 | − | x | x |
| = | - | 153600 | + | 800 | x | − | x | x |
| - | 153600 | + | 800 | x | − | x | x | = | 20000 |