| ( | 40 | − | 2 | x | ) | ( | 250 | − | 2 | x | ) | = | 7200 |
| 方程左边 = | 40 | ( | 250 | − | 2 | x | ) | − | 2 | x | ( | 250 | − | 2 | x | ) |
| = | 40 | × | 250 | − | 40 | × | 2 | x | − | 2 | x | ( | 250 | − | 2 | x | ) |
| = | 10000 | − | 80 | x | − | 2 | x | ( | 250 | − | 2 | x | ) |
| = | 10000 | − | 80 | x | − | 2 | x | × | 250 | + | 2 | x | × | 2 | x |
| = | 10000 | − | 80 | x | − | 500 | x | + | 4 | x | x |
| = | 10000 | − | 580 | x | + | 4 | x | x |
| 10000 | − | 580 | x | + | 4 | x | x | = | 7200 |