C :

#include<stdio.h>
#include<math.h>
#include<stdlib.h>
int sf(double a,double b,double c)
{
    double i;
    i=(b*b+a*a-c*c)/(2*b*a);
    if(i>(-1)&&i<1) return 1;
    else return 0;
}
double mj(double a,double b,double c)
{
    double p,m;
    p=(a+b+c)/2;
    m=sqrt(p*(p-a)*(p-b)*(p-c));
    return m;
    
}
int main()
{
    double a,b,c;
  //  scanf("%lf%lf%lf",&a,&b,&c);
   while(scanf("%lf%lf%lf",&a,&b,&c)==3)
    {
    if(sf(a,b,c)) printf("%.3lf",mj(a,b,c));
    else printf("data error!");
    printf("\n");
    }
      return 0;      
}

C++ :

	#include<iostream>
	#include<cstdio>
	#include<cmath>                                   
	using namespace std;
	double area(double,double,double);
	int main()
	{
	    for (int i=1; i<11; i++)
	    {
	        double a,b,c;
	        //cout<<"input a,b,c"<<endl;
	        cin>>a>>b>>c;
	        if ((a+b<=c)||(a+c<=b)||(b+c<=a))       
	        cout<<"data error!"<<endl; 
	        //判断三角形是否合法
	        else  printf("%0.3f\n",area(a,b,c));
	    }
	    return 0;
}

double area(double a,double b,double c)  //此函数为 海伦－秦九韶公式
{
    double p=(a+b+c)/2;
    return sqrt(p*(p-a)*(p-b)*(p-c));
}