| ( | 50 | − | 1 5 | x | ) | ( | 100 | + | x | ) | = | 0 |
| 方程左边 = | 50 | ( | 100 | + | x | ) | − | 1 5 | x | ( | 100 | + | x | ) |
| = | 50 | × | 100 | + | 50 | x | − | 1 5 | x | ( | 100 | + | x | ) |
| = | 5000 | + | 50 | x | − | 1 5 | x | ( | 100 | + | x | ) |
| = | 5000 | + | 50 | x | − | 1 5 | x | × | 100 | − | 1 5 | x | x |
| = | 5000 | + | 50 | x | − | 20 | x | − | 1 5 | x | x |
| = | 5000 | + | 30 | x | − | 1 5 | x | x |
| 5000 | + | 30 | x | − | 1 5 | x | x | = | 0 |