1.你这sum是末项啊,按你这么写输出应该是
```cpp
cout << (a + sum) * n / 2 << endl;
//首项+末项*项数/2
```
2.记得开long long(不然会寄,亲测)
by silent_ST @ 2023-07-20 08:27:53
附赠末项公式:末项=首项+(项数-1)* 公差
by silent_ST @ 2023-07-20 08:31:01