//正方形
void DoneSq(Point a, Point c)
{    //已知a c,求b d
     Point b, d;
     double x,y,mx, my;
     mx = (a.x+c.x)/2.0, my = (a.y+c.y)/2.0;     
     x = a.x - mx;    y = a.y - my;
     b.x = -y + mx;   b.y = x + my;
     x = c.x - mx;    y = c.y - my;
     d.x = - y + mx; d.y = x + my;
}