找规律，emmm没找出来，看的答案
#include<iostream>
#include<algorithm>
using namespace std;
const int maxn = 1e5 + 10;
double a[maxn];
int n;
int main()
{
double ans = 0;
scanf("%d", &n);
for (int i = 1; i <= n; i++)
{
scanf("%lf", &a[i]);
ans += a[i] * i*(n + 1 - i);
}
printf("%.2f", ans);
return 0;
}
