| ( | - | 10 | x | + | 700 | ) | ( | x | − | 20 | ) | = | 8750 |
| 方程左边 = | - | 10 | x | ( | x | − | 20 | ) | + | 700 | ( | x | − | 20 | ) |
| = | - | 10 | x | x | + | 10 | x | × | 20 | + | 700 | ( | x | − | 20 | ) |
| = | - | 10 | x | x | + | 200 | x | + | 700 | ( | x | − | 20 | ) |
| = | - | 10 | x | x | + | 200 | x | + | 700 | x | − | 700 | × | 20 |
| = | - | 10 | x | x | + | 200 | x | + | 700 | x | − | 14000 |
| = | - | 10 | x | x | + | 900 | x | − | 14000 |
| - | 10 | x | x | + | 900 | x | − | 14000 | = | 8750 |