程序代码：

    if(x1>x3)
     {wrt=atan((y1-y3)/(x1-x3));}
    if(x1<=x3)
     {wrt=pi+atan((y1-y3)/(x1-x3));}
    if(x1==x3]&&y1>=y3)
     {wtr=pi/2.0;}
    if(x1==x3&&y1<y3)
      {wrt=3.0*pi/2.0;}