题解 100% AC

#include<bits/stdc++.h>
using namespace std;
double ans(double a,double b,double c){
    double p=(a+b+c)/2.00;
    return sqrt(p*(p-a)*(p-b)*(p-c)*1.00);
}
int main(){
    double a,b,c;cin>>a>>b>>c;
    cout<<fixed<<setprecision(2)<<ans(a,b,c);
    return 0;
}