| ( | 50 | − | 2 | x | ) | ( | 40 | − | 3 | x | ) | = | 600 |
| 方程左边 = | 50 | ( | 40 | − | 3 | x | ) | − | 2 | x | ( | 40 | − | 3 | x | ) |
| = | 50 | × | 40 | − | 50 | × | 3 | x | − | 2 | x | ( | 40 | − | 3 | x | ) |
| = | 2000 | − | 150 | x | − | 2 | x | ( | 40 | − | 3 | x | ) |
| = | 2000 | − | 150 | x | − | 2 | x | × | 40 | + | 2 | x | × | 3 | x |
| = | 2000 | − | 150 | x | − | 80 | x | + | 6 | x | x |
| = | 2000 | − | 230 | x | + | 6 | x | x |
| 2000 | − | 230 | x | + | 6 | x | x | = | 600 |