x=(1-t)/(1+t)
x+tx=1-t
t(1+x)=1-x
t=(1-x)/(1+x)
y=2t/(1+t)
y+ty=2t
y=(2-y)t
t=y/(2-y)=(1-x)/(1+x)
即y/(2-y)=(1-x)/(1+x)