C :

# include <stdio.h>
# include <math.h>
int main()
{
	float a,b,c;
	float s,ar;
	while(scanf("%f%f%f",&a,&b,&c)!=EOF)
	{
		s=(a+b+c)/2;
		ar=sqrt(s*(s-a)*(s-b)*(s-c));
		printf("%.2f\n",ar);
	}
	return 0;
}

C++ :

#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <time.h>
int main()
{
  //  freopen("in","r",stdin);
 //   freopen("out","w",stdout);

    float a,b,c,s,area;

    while(scanf("%f%f%f",&a,&b,&c)!=EOF)
    {
       s=1.0/2*(a+b+c);
       area=sqrt(s*(s-a)*(s-b)*(s-c));
       printf("%.2f\n",area);
    }
  return 0;
}