------
## 最终结论
```python
def tn_gp(a, r, n):
    return a * (r ** (n - 1))

# Test cases
assert tn_gp(1, 5, 2) == 16
assert tn_gp(1, 5, 4) == 256
assert tn_gp(2, 6, 3) == 486
```