| ( | x | − | 30 | ) | ( | - | 2 | x | + | 240 | ) | = | 3600 |
| 方程左边 = | x | ( | - | 2 | x | + | 240 | ) | − | 30 | ( | - | 2 | x | + | 240 | ) |
| = | - | x | × | 2 | x | + | x | × | 240 | − | 30 | ( | - | 2 | x | + | 240 | ) |
| = | - | x | × | 2 | x | + | 240 | x | + | 30 | × | 2 | x | − | 30 | × | 240 |
| = | - | x | × | 2 | x | + | 240 | x | + | 60 | x | − | 7200 |
| = | - | x | × | 2 | x | + | 300 | x | − | 7200 |
| - | x | × | 2 | x | + | 300 | x | − | 7200 | = | 3600 |