可以不用printf

需要导入<iomanip>哦，或者像我一样，用万能头<bits/stdc++.h>

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