题解 | #牛牛学数列3#

#include <stdio.h>
#include <math.h>
int main() {
    double sum = 0, tempSum = 0;
    int n;
    while (scanf("%d", &n) != EOF) {
        for (int i = 1; i <= n; i++) {
            tempSum += pow((-1), (i - 1)) * (2 * i - 1); // 先把分母求和
            sum += 1.0 / tempSum;
        }
    }
    printf("%.3lf\n", sum);

    return 0;
}