| ( | x | − | 30 | ) | ( | 700 | − | 10 | x | ) | − | 150 | = | 3600 |
| 方程左边 = | x | ( | 700 | − | 10 | x | ) | − | 30 | ( | 700 | − | 10 | x | ) | − | 150 |
| = | x | × | 700 | − | x | × | 10 | x | − | 30 | ( | 700 | − | 10 | x | ) | − | 150 |
| = | 700 | x | − | x | × | 10 | x | − | 30 | × | 700 | + | 30 | × | 10 | x | − | 150 |
| = | 700 | x | − | x | × | 10 | x | − | 21000 | + | 300 | x | − | 150 |
| = | 1000 | x | − | x | × | 10 | x | − | 21150 |
| 1000 | x | − | x | × | 10 | x | − | 21150 | = | 3600 |