| ( | x | + | 98 | ) | ( | 42 | − | x | ) | = | 25 | × | 640 |
| 方程左边 = | x | ( | 42 | − | x | ) | + | 98 | ( | 42 | − | x | ) |
| = | x | × | 42 | − | x | x | + | 98 | ( | 42 | − | x | ) |
| = | 42 | x | − | x | x | + | 98 | × | 42 | − | 98 | x |
| = | 42 | x | − | x | x | + | 4116 | − | 98 | x |
| = | - | 56 | x | − | x | x | + | 4116 |
| - | 56 | x | − | x | x | + | 4116 | = | 25 | × | 640 |
| 方程右边 = | 16000 |
| - | 56 | x | − | x | x | + | 4116 | = | 16000 |