| ( | x | − | 30 | ) | ( | - | 15 | x | + | 1500 | ) | = | 900 |
| 方程左边 = | x | ( | - | 15 | x | + | 1500 | ) | − | 30 | ( | - | 15 | x | + | 1500 | ) |
| = | - | x | × | 15 | x | + | x | × | 1500 | − | 30 | ( | - | 15 | x | + | 1500 | ) |
| = | - | x | × | 15 | x | + | 1500 | x | + | 30 | × | 15 | x | − | 30 | × | 1500 |
| = | - | x | × | 15 | x | + | 1500 | x | + | 450 | x | − | 45000 |
| = | - | x | × | 15 | x | + | 1950 | x | − | 45000 |
| - | x | × | 15 | x | + | 1950 | x | − | 45000 | = | 900 |