------
## 最终结论
```python
import math

def area_pentagon(side_length):
    area = (5 * side_length ** 2) / (4 * math.tan(math.pi / 5))
    return area

# Test cases
assert area_pentagon(5) == 43.01193501472417
assert area_pentagon(10) == 172.0477400588967
assert area_pentagon(15) == 387.10741513251753
```