| ( | 25 | − | X | ) | ( | - | 500 | X | + | 20000 | ) | = | 0 |
| 方程左边 = | 25 | ( | - | 500 | X | + | 20000 | ) | − | X | ( | - | 500 | X | + | 20000 | ) |
| = | - | 25 | × | 500 | X | + | 25 | × | 20000 | − | X | ( | - | 500 | X | + | 20000 | ) |
| = | - | 12500 | X | + | 500000 | − | X | ( | - | 500 | X | + | 20000 | ) |
| = | - | 12500 | X | + | 500000 | + | X | × | 500 | X | − | X | × | 20000 |
| = | - | 32500 | X | + | 500000 | + | X | × | 500 | X |
| - | 32500 | X | + | 500000 | + | X | × | 500 | X | = | 0 |