C :

#include<stdio.h>
#include<math.h>
int main()
{
	double a,b,c,area,p;
	scanf("%lf%lf%lf",&a,&b,&c);
	p=(a+b+c)/2;
	area=sqrt(p*(p-a)*(p-b)*(p-c));
	printf("area=%.6f\n",area);
	return 0;
}

C++ :

#include<cstdio>
#include<cmath>
int main()
{
	double a,b,c,p,area;
	scanf("%lf%lf%lf",&a,&b,&c);
	p=1.0/2*(a+b+c);
	area=sqrt(p*(p-a)*(p-b)*(p-c));
	printf("area=%lf\n",area);
	return 0;
}