[2020.10.28 SSL Simulation Tournament T3] Sum of Interval Sum [Number Theory]

analysis:

Find the formula:
for $i$包含$a\left[i\right]$ Imaginary unit sharing$i\ast (n-i+1)$ one of the$a\left[i\right]$ Completion of calculation$i\ast (n-i+1)$ times
lastaccumulateto

CODE：

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
const long long p=1e9+7;
long long n,x,ans;
int main(){
    
    
	scanf("%lld",&n); 
	for(long long i=1;i<=n;i++)
	{
    
    
		scanf("%lld",&x);
		ans=(ans+x*(i*(n-i+1)))%p;  //累加 公式
	}
	printf("%lld",ans%p);
	
	return 0;
}