题解 | #等差数列#

1. 等差数列得通项公式 ai = a1+ (i-1)*d;

   #include<bits/stdc++.h>
   

using namespace std;

int main(){

int in;

while(cin>>in){
    int a1 = 2, d = 3;
    int sum = 0;


    for(int i = 1; i <= in;i++){
        sum += a1+ (i-1)*d;
    }

    cout<<sum<<endl;


}

return 0;

}
```