Acperience (英语阅读 + 数学推导)

#include<bits/stdc++.h>
using namespace std;

int main(){
    int T,n,m;scanf("%d",&T);
    while(T --){
        scanf("%d",&n);
        long long nksum = 0, sum = 0;
        for(int i = 0; i < n; i ++){
            scanf("%d", &m);
            nksum += 1LL * n * m * m;
            sum += abs(1LL * m);
        }
        long long a = nksum - sum * sum;
        long long b = n;
        long long gcd = __gcd(a, b);
        printf("%lld/%lld\n",a/gcd, b/gcd);
    }
    return 0;
}