题解 | #等差数列#抛砖引玉
等差数列
http://www.nowcoder.com/practice/f792cb014ed0474fb8f53389e7d9c07f
#include<iostream>
using namespace std;
int main() {
int n;
cin >> n;
int sum = 0;
for (int i = 1; i <= n; i++) {
sum += (3 * i - 1);
}
cout << sum << endl;
return 0;
}
