| 150 | = | ( | - | 2 | x | + | 80 | ) | ( | x | − | 20 | ) |
| 方程右边 = | - | 2 | x | ( | x | − | 20 | ) | + | 80 | ( | x | − | 20 | ) |
| = | - | 2 | x | x | + | 2 | x | × | 20 | + | 80 | ( | x | − | 20 | ) |
| = | - | 2 | x | x | + | 40 | x | + | 80 | ( | x | − | 20 | ) |
| = | - | 2 | x | x | + | 40 | x | + | 80 | x | − | 80 | × | 20 |
| = | - | 2 | x | x | + | 40 | x | + | 80 | x | − | 1600 |
| = | - | 2 | x | x | + | 120 | x | − | 1600 |
| 150 | = | - | 2 | x | x | + | 120 | x | − | 1600 |