| ( | 200 | − | 10 | x | ) | ( | 5 | + | x | ) | = | 1840 |
| 方程左边 = | 200 | ( | 5 | + | x | ) | − | 10 | x | ( | 5 | + | x | ) |
| = | 200 | × | 5 | + | 200 | x | − | 10 | x | ( | 5 | + | x | ) |
| = | 1000 | + | 200 | x | − | 10 | x | ( | 5 | + | x | ) |
| = | 1000 | + | 200 | x | − | 10 | x | × | 5 | − | 10 | x | x |
| = | 1000 | + | 200 | x | − | 50 | x | − | 10 | x | x |
| = | 1000 | + | 150 | x | − | 10 | x | x |
| 1000 | + | 150 | x | − | 10 | x | x | = | 1840 |