| ( | 40 | − | 2 | x | ) | ( | 30 | − | 2 | x | ) | = | 525 |
| 方程左边 = | 40 | ( | 30 | − | 2 | x | ) | − | 2 | x | ( | 30 | − | 2 | x | ) |
| = | 40 | × | 30 | − | 40 | × | 2 | x | − | 2 | x | ( | 30 | − | 2 | x | ) |
| = | 1200 | − | 80 | x | − | 2 | x | ( | 30 | − | 2 | x | ) |
| = | 1200 | − | 80 | x | − | 2 | x | × | 30 | + | 2 | x | × | 2 | x |
| = | 1200 | − | 80 | x | − | 60 | x | + | 4 | x | x |
| = | 1200 | − | 140 | x | + | 4 | x | x |
| 1200 | − | 140 | x | + | 4 | x | x | = | 525 |