测试
小标题
\[\sum \limits_{k=1}^n A_k
\]
#include <bits/stdc++.h>
using namespace std;
int main() {
cout << "hello world" << endl;
return 0;
}
#include <bits/stdc++.h>
using namespace std;
int main() {
cout << "hello world" << endl;
return 0;
}