| ( | 50 | − | x | ) | ( | 300 | + | 300 | x | ) | = | 0 |
| 方程左边 = | 50 | ( | 300 | + | 300 | x | ) | − | x | ( | 300 | + | 300 | x | ) |
| = | 50 | × | 300 | + | 50 | × | 300 | x | − | x | ( | 300 | + | 300 | x | ) |
| = | 15000 | + | 15000 | x | − | x | ( | 300 | + | 300 | x | ) |
| = | 15000 | + | 15000 | x | − | x | × | 300 | − | x | × | 300 | x |
| = | 15000 | + | 14700 | x | − | x | × | 300 | x |
| 15000 | + | 14700 | x | − | x | × | 300 | x | = | 0 |