| ( | X | + | 200 | ) | ( | X | + | 200 | ) | − | X | X | = | 200000 |
| 方程左边 = | X | ( | X | + | 200 | ) | + | 200 | ( | X | + | 200 | ) | − | X | X |
| = | X | X | + | X | × | 200 | + | 200 | ( | X | + | 200 | ) | − | X | X |
| = | X | X | + | 200 | X | + | 200 | X | + | 200 | × | 200 | − | X | X |
| = | X | X | + | 200 | X | + | 200 | X | + | 40000 | − | X | X |
| = | X | X | + | 400 | X | + | 40000 | − | X | X |
| X | X | + | 400 | X | + | 40000 | − | X | X | = | 200000 |