| - | 200 | ( | x | − | 30 | ) | ( | x | − | 43 | ) | = | 8400 |
| 方程左边 = | - | 200 | x | ( | x | − | 43 | ) | + | 200 | × | 30 | ( | x | − | 43 | ) |
| = | - | 200 | x | ( | x | − | 43 | ) | + | 6000 | ( | x | − | 43 | ) |
| = | - | 200 | x | x | + | 200 | x | × | 43 | + | 6000 | ( | x | − | 43 | ) |
| = | - | 200 | x | x | + | 8600 | x | + | 6000 | ( | x | − | 43 | ) |
| = | - | 200 | x | x | + | 8600 | x | + | 6000 | x | − | 6000 | × | 43 |
| = | - | 200 | x | x | + | 8600 | x | + | 6000 | x | − | 258000 |
| = | - | 200 | x | x | + | 14600 | x | − | 258000 |
| - | 200 | x | x | + | 14600 | x | − | 258000 | = | 8400 |