C :

#include"stdio.h"
#include"math.h"
float bianchang(float x1,float x2,float y1,float y2)
{
    return (sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2)));
}
int main()
{
    float x1,x2,x3,y1,y2,y3;
    while(1)
    {
        scanf("%f%f%f%f%f%f",&x1,&y1,&x2,&y2,&x3,&y3);
        float a,b,c,p;
        a=bianchang(x1,x2,y1,y2);b=bianchang(x1,x3,y1,y3);
        c=bianchang(x2,x3,y2,y3);
        if(a==0&&b==0&&c==0)
            break;
        p=(a+b+c)/2;
        float s;
        s=sqrt(p*(p-a)*(p-b)*(p-c));
        printf("%.1f\n",s);
    }
    return 0;
}

C++ :

#include<stdio.h>
#include<math.h>
int main()
{
  float x1,y1,x2,y2,x3,y3,a,b,c,p,s;
  while(scanf("%f%f%f%f%f%f",&x1,&y1,&x2,&y2,&x3,&y3))
  { if(x1==0&&y1==0&&x2==0&&y2==0&&x3==0&&y3==0) break;
  a=sqrt(1.0*(x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
  b=sqrt((x3-x2)*(x3-x2)+(y3-y2)*(y3-y2));
  c=sqrt((x3-x1)*(x3-x1)+(y3-y1)*(y3-y1));
  p=(a+b+c)/2.0;
  s=sqrt(p*(p-a)*(p-b)*(p-c));
  printf("%0.1f\n",s);
  }
  return 0;
}

Java :

import java.util.Scanner;

public class Main
{
	public static void main(String[] args)
	{
		Scanner sc=new Scanner(System.in);
		while(sc.hasNext())
		{
			int x1=sc.nextInt();
			int y1=sc.nextInt();
			int x2=sc.nextInt();
			int y2=sc.nextInt();
			int x3=sc.nextInt();
			int y3=sc.nextInt();
			if(x1==0&&y1==0&&x2==0&&y2==0&&x3==0&y3==0)
				break;
			getAreas(x1,y1,x2,y2,x3,y3);
		}
	}
	public static void getAreas(int x1,int y1,int x2,int y2,int x3,int y3)
	{
		double ares=Math.abs((double)((x1*y2+x3*y1+x2*y3-x3*y2-x1*y3-x2*y1)/2.0));
		System.out.printf("%.1f",ares);
		System.out.println();
	}
}