| ( | - | 2 | x | + | 120 | ) | ( | x | − | 20 | ) | = | 600 |
| 方程左边 = | - | 2 | x | ( | x | − | 20 | ) | + | 120 | ( | x | − | 20 | ) |
| = | - | 2 | x | x | + | 2 | x | × | 20 | + | 120 | ( | x | − | 20 | ) |
| = | - | 2 | x | x | + | 40 | x | + | 120 | ( | x | − | 20 | ) |
| = | - | 2 | x | x | + | 40 | x | + | 120 | x | − | 120 | × | 20 |
| = | - | 2 | x | x | + | 40 | x | + | 120 | x | − | 2400 |
| = | - | 2 | x | x | + | 160 | x | − | 2400 |
| - | 2 | x | x | + | 160 | x | − | 2400 | = | 600 |