小白赛j

题目要我们求得答案为

我们令
由于 矩阵的对称性知

#include<cstdio>
#include<iostream>
using namespace std;
typedef long long ll;
const int mod =1e9+7;
ll a[500005];
int main(){
    int n;
    scanf("%d",&n);
    ll sum1=0,sum2=0;
    for(int i=1;i<=n;++i){
        scanf("%d",&a[i]);
        sum1=(sum1+a[i]*a[i])%mod;
        sum2=(sum2+a[i])%mod;
    }
    ll ans=((n*sum1-sum2*sum2)%mod+mod)%mod;
    cout<<ans<<endl;
    return 0;
}