| 400 | = | ( | 120 | − | 2 | x | ) | ( | x | − | 49 | ) |
| 方程右边 = | 120 | ( | x | − | 49 | ) | − | 2 | x | ( | x | − | 49 | ) |
| = | 120 | x | − | 120 | × | 49 | − | 2 | x | ( | x | − | 49 | ) |
| = | 120 | x | − | 5880 | − | 2 | x | ( | x | − | 49 | ) |
| = | 120 | x | − | 5880 | − | 2 | x | x | + | 2 | x | × | 49 |
| = | 120 | x | − | 5880 | − | 2 | x | x | + | 98 | x |
| = | 218 | x | − | 5880 | − | 2 | x | x |
| 400 | = | 218 | x | − | 5880 | − | 2 | x | x |