| ( | x | − | 50 | ) | ( | - | 20 | x | + | 2600 | ) | = | 0 |
| 方程左边 = | x | ( | - | 20 | x | + | 2600 | ) | − | 50 | ( | - | 20 | x | + | 2600 | ) |
| = | - | x | × | 20 | x | + | x | × | 2600 | − | 50 | ( | - | 20 | x | + | 2600 | ) |
| = | - | x | × | 20 | x | + | 2600 | x | + | 50 | × | 20 | x | − | 50 | × | 2600 |
| = | - | x | × | 20 | x | + | 2600 | x | + | 1000 | x | − | 130000 |
| = | - | x | × | 20 | x | + | 3600 | x | − | 130000 |
| - | x | × | 20 | x | + | 3600 | x | − | 130000 | = | 0 |