nyoj-815-三角形

#include<stdio.h>
#include<math.h>
int main()
{
    int s;
    scanf("%d",&s);
    while(s--)
    {
        double x1,x2,x3,y1,y2,y3,a,b,c;
        scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x3,&y3);
        a=sqrt((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));
        if(a+b>c&&a+c>b&&b+c>a)
        printf("%.3lf\n",(sqrt((a+b+c)*(a+b-c)*(a+c-b)*(b+c-a)))/4);
        else
        printf("Can not form a triangle.\n");
    }
    return 0;
}