| ( | 50 | − | 2 | x | ) | ( | 30 | − | 2 | x | ) | = | 300 |
| 方程左边 = | 50 | ( | 30 | − | 2 | x | ) | − | 2 | x | ( | 30 | − | 2 | x | ) |
| = | 50 | × | 30 | − | 50 | × | 2 | x | − | 2 | x | ( | 30 | − | 2 | x | ) |
| = | 1500 | − | 100 | x | − | 2 | x | ( | 30 | − | 2 | x | ) |
| = | 1500 | − | 100 | x | − | 2 | x | × | 30 | + | 2 | x | × | 2 | x |
| = | 1500 | − | 100 | x | − | 60 | x | + | 4 | x | x |
| = | 1500 | − | 160 | x | + | 4 | x | x |
| 1500 | − | 160 | x | + | 4 | x | x | = | 300 |