| 90000 | = | ( | 400 | − | x | ) | ( | 400 | − | x | ) | − | x | x |
| 方程右边 = | 400 | ( | 400 | − | x | ) | − | x | ( | 400 | − | x | ) | − | x | x |
| = | 400 | × | 400 | − | 400 | x | − | x | ( | 400 | − | x | ) | − | x | x |
| = | 160000 | − | 400 | x | − | x | ( | 400 | − | x | ) | − | x | x |
| = | 160000 | − | 400 | x | − | x | × | 400 | + | x | x | − | x | x |
| = | 160000 | − | 800 | x | + | x | x | − | x | x |
| 90000 | = | 160000 | − | 800 | x | + | x | x | − | x | x |
| x= | 175 2 |