y=2x/(x-1)
=[2(x-1)+2]/(x-1)
=2+2/(x-1)
y-2=2/(x-1)
x-1=2/(y-2)
x=2/(y-2) +1=y/(y-2)
所以f(x)=2x/(x-1) 的反函数为：
f'(x)=x/(x-2)