| 4 | ( | 9 | ÷ | 2 | ) | − | a | ÷ | 2 | = | - | 2 | ( | 9 | ÷ | 2 | − | 1 | ) |
| 方程左边 = | 4 | × | 9 | ÷ | 2 | − | 1 2 | a |
| = | 18 | − | 1 2 | a |
| 18 | − | 1 2 | a | = | - | 2 | ( | 9 | ÷ | 2 | − | 1 | ) |
| 方程右边 = | - | 2 | × | 9 | ÷ | 2 | + | 2 | × | 1 |
| = | - | 9 | + | 2 |
| = | - | 7 |
| 18 | − | 1 2 | a | = | - | 7 |
| - | 1 2 | a | = | - | 7 | − | 18 |
| - | 1 2 | a | = | - | 25 |
| 25 | = | 1 2 | a |
1 2 | a | = | 25 |
| a | = | 25 | ÷ | 1 2 |
| = | 25 | × | 2 |
| a | = | 50 |