题解 | #牛牛学数列3#

#include <iostream>
using namespace std;
#include <bits/stdc++.h>

int main() {
    int n = 0;              //定义变量
    int denominator = 0;    //分母
    double sum = 0;
    cin >> n;

    for (int i = 1; i <= n; i++) {
        //计算分母，当i为奇数
        if (i % 2 == 1) {
            denominator += 2 * i - 1;
        }
        //i位偶数
        else {
            denominator -= 2 * i - 1;
        }

        //计算总和
        sum += 1.0 / denominator;
    }

    cout << fixed << setprecision(3) << sum;

    system("pause");
    return 0;
}