| ( | x | − | 40 | ) | ( | - | 2 | x | + | 220 | ) | = | 2250 |
| 方程左边 = | x | ( | - | 2 | x | + | 220 | ) | − | 40 | ( | - | 2 | x | + | 220 | ) |
| = | - | x | × | 2 | x | + | x | × | 220 | − | 40 | ( | - | 2 | x | + | 220 | ) |
| = | - | x | × | 2 | x | + | 220 | x | + | 40 | × | 2 | x | − | 40 | × | 220 |
| = | - | x | × | 2 | x | + | 220 | x | + | 80 | x | − | 8800 |
| = | - | x | × | 2 | x | + | 300 | x | − | 8800 |
| - | x | × | 2 | x | + | 300 | x | − | 8800 | = | 2250 |