| - | 10 | ( | x | − | 30 | ) | ( | x | − | 30 | ) | + | 4000 | = | 0 |
| 方程左边 = | - | 10 | x | ( | x | − | 30 | ) | + | 10 | × | 30 | ( | x | − | 30 | ) | + | 4000 |
| = | - | 10 | x | ( | x | − | 30 | ) | + | 300 | ( | x | − | 30 | ) | + | 4000 |
| = | - | 10 | x | x | + | 10 | x | × | 30 | + | 300 | ( | x | − | 30 | ) | + | 4000 |
| = | - | 10 | x | x | + | 300 | x | + | 300 | ( | x | − | 30 | ) | + | 4000 |
| = | - | 10 | x | x | + | 300 | x | + | 300 | x | − | 300 | × | 30 | + | 4000 |
| = | - | 10 | x | x | + | 300 | x | + | 300 | x | − | 9000 | + | 4000 |
| = | - | 10 | x | x | + | 600 | x | − | 5000 |
| - | 10 | x | x | + | 600 | x | − | 5000 | = | 0 |