#include <iostream>
using namespace std;
double harmonicMean(double x, double y);
int main() {
double x, y;
while (cin >> x >> y) { // 这一步是读取一行的两个数
if (0 == x || 0 == y) break;
cout << harmonicMean(x, y) << endl;
}
return 0;
}
double harmonicMean(double x, double y) {
return 2.0 * x * y / (x + y);
}
原文：https://www.cnblogs.com/ranwuer/p/9716230.html