| ( | x | − | 120 | ) | ( | 120 | − | ( | x | − | 120 | ) | ) | = | 3200 |
| 方程左边 = | x | ( | 120 | − | ( | x | − | 120 | ) | ) | − | 120 | ( | 120 | − | ( | x | − | 120 | ) | ) |
| = | x | × | 120 | − | x | ( | x | − | 120 | ) | − | 120 | ( | 120 | − | ( | x | − | 120 | ) | ) |
| = | 120 | x | − | x | x | + | x | × | 120 | − | 120 | ( | 120 | − | ( | x | − | 120 | ) | ) |
| = | 240 | x | − | x | x | − | 120 | ( | 120 | − | ( | x | − | 120 | ) | ) |
| = | 240 | x | − | x | x | − | 120 | × | 120 | + | 120 | ( | x | − | 120 | ) |
| = | 240 | x | − | x | x | − | 14400 | + | 120 | ( | x | − | 120 | ) |
| = | 240 | x | − | x | x | − | 14400 | + | 120 | x | − | 120 | × | 120 |
| = | 240 | x | − | x | x | − | 14400 | + | 120 | x | − | 14400 |
| = | 360 | x | − | x | x | − | 28800 |
| 360 | x | − | x | x | − | 28800 | = | 3200 |