| 30 | ( | x | − | 6 | ) | ( | 25 | − | x | ) | = | 6300 |
| 方程左边 = | 30 | x | ( | 25 | − | x | ) | − | 30 | × | 6 | ( | 25 | − | x | ) |
| = | 30 | x | ( | 25 | − | x | ) | − | 180 | ( | 25 | − | x | ) |
| = | 30 | x | × | 25 | − | 30 | x | x | − | 180 | ( | 25 | − | x | ) |
| = | 750 | x | − | 30 | x | x | − | 180 | ( | 25 | − | x | ) |
| = | 750 | x | − | 30 | x | x | − | 180 | × | 25 | + | 180 | x |
| = | 750 | x | − | 30 | x | x | − | 4500 | + | 180 | x |
| = | 930 | x | − | 30 | x | x | − | 4500 |
| 930 | x | − | 30 | x | x | − | 4500 | = | 6300 |