题解 | 等差数列
#include <iostream>
using namespace std;
int main() {
int n;
cin >> n;
int begin = 2;
int end = 3*n-1;
cout << (begin + end)*n/2;//(首项+末项)*项数/2
}
// 64 位输出请用 printf("%lld")
#include <iostream>
using namespace std;
int main() {
int n;
cin >> n;
int begin = 2;
int end = 3*n-1;
cout << (begin + end)*n/2;//(首项+末项)*项数/2
}
// 64 位输出请用 printf("%lld")
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