按照公式，注意输出

#include<iostream>
#include<cmath>
using namespace std;
double area(double a,double b,double c){
double p=0.5*(a+b+c);
return sqrt(p*(p-a)(p-b)(p-c));

}
int main(){
double a=0,b=0,c=0;
cin>>a>>b>>c;
printf("%.2lf",area(a,b,c));

return 0;

}