就是要把m消去
x=2m/(1+m^2)+1
(x-1)/2=m/(1+m^2).(1)
y=(1-m^2)/(1+m^2)
(y+1)/2=1/(1+m^2).(2)
(1)/(2),有：
(x-1)/(y+1)=m.(3)
(1)*(3)+(2),有：
(x-1)/2*(x-1)/(y+1)+(y+1)/2=1
即 (x-1)^2+(y+1)^2=2(y+1)
(x-1)^2+y^2=1