自定义一个函数求1+1/2+1/3+……+1/n，编写程序，在主函数中求s=s1+s2+s3的值，其中： s1=1+1/2+1/3+……+1/5 s2=1+1/2+1/3+……+1/10 s3=1+1/2+1/3+……+1/20

您可以自定义一个函数来求1+1/2+1/3+……+1/n，代码如下：

double sum(int n) {
    double res = 0;
    for (int i = 1; i <= n; i++) {
        res += 1.0 / i;
    }
    return res;
}

然后在主函数中，您可以调用这个函数来求s1、s2、s3的值，代码如下：

#include <iostream>
using namespace std;

double sum(int n);

int main() {
    double s1 = sum(5);
    double s2 = sum(10);
    double s3 = sum(20);
    double s = s1 + s2 + s3;
    cout << "s1 = " << s1 << endl;
    cout << "s2 = " << s2 << endl;
    cout << "s3 = " << s3 << endl;
    cout << "s = " << s << endl;
    return 0;
}

double sum(int n) {
    double res = 0;
    for (int i = 1; i <= n; i++) {
        res += 1.0 / i;
    }
    return res;
}

输出结果为：

s1 = 2.28333
s2 = 2.92897
s3 = 3.59774
s = 8.81004