| ( | 100 | − | 2 | x | ) | ( | 90 | − | x | ) | = | 8448 |
| 方程左边 = | 100 | ( | 90 | − | x | ) | − | 2 | x | ( | 90 | − | x | ) |
| = | 100 | × | 90 | − | 100 | x | − | 2 | x | ( | 90 | − | x | ) |
| = | 9000 | − | 100 | x | − | 2 | x | ( | 90 | − | x | ) |
| = | 9000 | − | 100 | x | − | 2 | x | × | 90 | + | 2 | x | x |
| = | 9000 | − | 100 | x | − | 180 | x | + | 2 | x | x |
| = | 9000 | − | 280 | x | + | 2 | x | x |
| 9000 | − | 280 | x | + | 2 | x | x | = | 8448 |