两行代码

#include<bits/stdc++.h>
double s(double a,double b,double c){double p=(a+b+c)/2.0;return sqrt(p*(p-a)*(p-b)*(p-c));}
int main(){double a,b,c;std::cout.precision(2);std::cin>>a>>b>>c;printf("%.2f",s(a,b,c));return 0;}