数论片段和

题源

PTA 乙级

题解

#include<iostream>
#include<cstdio>
using namespace std;
int main() { 
	int n;
	long double num, sum = 0.0;
	cin >> n;
	for (int i = 1; i <= n; i++) {
		cin >> num;
		sum += (num * i) * (n - i + 1);
	}
	printf("%.2llf", sum);
	return 0;
}