| 80 | = | ( | - | 10 | x | + | 80 | ) | ( | x | − | 2 | ) |
| 方程右边 = | - | 10 | x | ( | x | − | 2 | ) | + | 80 | ( | x | − | 2 | ) |
| = | - | 10 | x | x | + | 10 | x | × | 2 | + | 80 | ( | x | − | 2 | ) |
| = | - | 10 | x | x | + | 20 | x | + | 80 | ( | x | − | 2 | ) |
| = | - | 10 | x | x | + | 20 | x | + | 80 | x | − | 80 | × | 2 |
| = | - | 10 | x | x | + | 20 | x | + | 80 | x | − | 160 |
| = | - | 10 | x | x | + | 100 | x | − | 160 |
| 80 | = | - | 10 | x | x | + | 100 | x | − | 160 |