| ( | 27 | − | x | ) | ( | 21 | − | x | ) | = | 27 | × | 21 | × | 3 4 |
| 方程左边 = | 27 | ( | 21 | − | x | ) | − | x | ( | 21 | − | x | ) |
| = | 27 | × | 21 | − | 27 | x | − | x | ( | 21 | − | x | ) |
| = | 567 | − | 27 | x | − | x | ( | 21 | − | x | ) |
| = | 567 | − | 27 | x | − | x | × | 21 | + | x | x |
| = | 567 | − | 48 | x | + | x | x |
| 567 | − | 48 | x | + | x | x | = | 27 | × | 21 | × | 3 4 |
| 方程右边 = | 1701 4 |
| 567 | − | 48 | x | + | x | x | = | 1701 4 |