链接：https://ac.nowcoder.com/acm/contest/881/B
来源：牛客网
 

 

裂项，化乘为加

 

 以此类推

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<string>
#include<cstring>
#include<map>
#include<vector>
#include<set>
#include<list>
#include<stack>
#include<queue>
using namespace std;
typedef long long ll;
const int mod=1e9+7;
ll a[1005];
int n;
ll ksm(ll a,ll b){
	ll s=1;
	while(b){
		if(b&1){
			s%=mod;
			a%=mod;
			s*=a;
		}
		a%=mod;
		a*=a;
		b>>=1;
	}
	return s%=mod;
}
ll k(int x){
	ll ans=1;
	for(int i=1;i<=n;i++){
		if(i!=x){
			ans*=(a[i]*a[i]-a[x]*a[x])%mod;
			ans=(ans%mod+mod)%mod;
		}
	}
	return ans;
}
int main(){
    while(~scanf("%d",&n)){
    	for(int i=1;i<=n;i++){
    		scanf("%d",&a[i]);
		}
		ll ans=0;
		for(int i=1;i<=n;i++){
			ans+=ksm(k(i)*(a[i]<<1)%mod,mod-2)%mod;
			ans%=mod; 
		} 
		printf("%lld\n",ans);
	}
	return 0;
}