------
## 最终结论
```python
def sum_gp(a, n, r):
    if r == 1:
        return a * n
    return a * (r**n - 1) // (r - 1)

assert sum_gp(1,5,2)==31
assert sum_gp(1,5,4)==341
assert sum_gp(2,6,3)==728
```