| ( | 800 | ÷ | 2 | ) | − | X | = | ( | 800 | − | X | ) | ÷ | 2 |
| 方程左边 = | 800 | ÷ | 2 | − | X |
| = | 400 | − | X |
| 400 | − | X | = | ( | 800 | − | X | ) | ÷ | 2 |
| 方程右边 = | 800 | × | 1 2 | − | X | × | 1 2 |
| = | 400 | − | X | × | 1 2 |
| 400 | − | X | = | 400 | − | 1 2 | X |
| - | X | + | 1 2 | X | = | 400 | − | 400 |
| - | 1 2 | X | = | 400 | − | 400 |
| - | 1 2 | X | = | 0 |
| - | 0 | = | 1 2 | X |
1 2 | X | = | - | 0 |
| X | = | - | 0 | ÷ | 1 2 |
| = | - | 0 | × | 2 |
| X | = | 0 |