三角形的面积

double count_triangle_area(Point a, Point b, Point c) {
    double area = -1;
    double side[3];
　　 side[0] = sqrt(pow(a.x - b.x, 2) + pow(a.y - b.y, 2));

    side[1] = sqrt(pow(a.x - z.x, 2) + pow(a.y - z.y, 2));

    side[2] = sqrt(pow(b.x - z.x, 2) + pow(b.y - z.y, 2)); 
  //side[0] = sqrt(pow(a.x - b.x, 2) + pow(a.y - b.y, 2) + pow(a.z - b.z, 2));
  //side[1] = sqrt(pow(a.x - c.x, 2) + pow(a.y - c.y, 2) + pow(a.z - c.z, 2));
  //side[2] = sqrt(pow(c.x - b.x, 2) + pow(c.y - b.y, 2) + pow(c.z - b.z, 2)); 

   if (side[0] + side[1] <= side[2] || side[0] + side[2] <= side[1] || side[1] + side[2] <= side[0]) 
        return area;
   double p = (side[0] + side[1] + side[2]) / 2;
   area = sqrt(p*(p - side[0])*(p - side[1])*(p - side[2]));
   return area; 
}