答：
y=g(x)=2(x+1)/(x-1)
=2(x-1+2)/(x-1)
=2+4/(x-1)
y-2=4/(x-1)
x-1=4/(y-2)
所以：
g-1(x)=1+4/(x-2)=(x+2)/(x-2)
因为：
f(x)=lnx
所以：f [g-1(x) ]=ln [(x+2)/(x-2)]