| ( | 5 2 | + | x | ) | ( | 80000 | − | 2000 | x | ) | = | 200000 |
| 方程左边 = | 5 2 | ( | 80000 | − | 2000 | x | ) | + | x | ( | 80000 | − | 2000 | x | ) |
| = | 5 2 | × | 80000 | − | 5 2 | × | 2000 | x | + | x | ( | 80000 | − | 2000 | x | ) |
| = | 200000 | − | 5000 | x | + | x | ( | 80000 | − | 2000 | x | ) |
| = | 200000 | − | 5000 | x | + | x | × | 80000 | − | x | × | 2000 | x |
| = | 200000 | + | 75000 | x | − | x | × | 2000 | x |
| 200000 | + | 75000 | x | − | x | × | 2000 | x | = | 200000 |
| x2= | 75 2 |