6561 25 | = | 400 | ( | 1 | + | r | ) | − | 4 |
| 方程右边 = | 400 | × | 1 | + | 400 | r | − | 4 |
| = | 400 | + | 400 | r | − | 4 |
| = | 396 | + | 400 | r |
6561 25 | = | 396 | + | 400 | r |
| - | 400 | r | = | 396 | − | 6561 25 |
| - | 400 | r | = | 3339 25 |
| - | 3339 25 | = | 400 | r |
| 400 | r | = | - | 3339 25 |
| r | = | - | 3339 25 | ÷ | 400 |
| = | - | 3339 25 | × | 1 400 |
| r | = | - | 3339 10000 |
| r | = | - 0.3339 |