| ( | 8 | + | 25 2 | x | ) | ( | 400 | − | x | ) | = | 5000 |
| 方程左边 = | 8 | ( | 400 | − | x | ) | + | 25 2 | x | ( | 400 | − | x | ) |
| = | 8 | × | 400 | − | 8 | x | + | 25 2 | x | ( | 400 | − | x | ) |
| = | 3200 | − | 8 | x | + | 25 2 | x | ( | 400 | − | x | ) |
| = | 3200 | − | 8 | x | + | 25 2 | x | × | 400 | − | 25 2 | x | x |
| = | 3200 | − | 8 | x | + | 5000 | x | − | 25 2 | x | x |
| = | 3200 | + | 4992 | x | − | 25 2 | x | x |
| 3200 | + | 4992 | x | − | 25 2 | x | x | = | 5000 |