题解 | #牛牛学数列3#

#include <stdio.h>
#include <math.h>

int main() {

    int n = 0;//定义项数
    scanf("%d", &n);
    int i = 0;//遍历
    int x = 1;
    int t = 0;
    double sum = 0;
    for (i = 1; i <= n; i++)//项数
    {
       t += (2*i-1)*x;
       sum += 1.0/t;
       x = -x;
    }
    printf("%.3lf\n", sum);
    return 0;
}