用sprt函数

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