#include<bits/stdc++.h>
using namespace std;
double f,u;
int main(){
	cin>>f>>u;
	printf("%.2f",(f*u)/(u-f));
} 

查了亿下公式

#include<bits/stdc++.h>
using namespace std;
float f,u;

int main(){
	cin>>f>>u;
	printf("%.2f",(f*u)/(u-f));
} 

hhhhhhhhhhhhhhh