取x=2/t-1
则f(t)=f(2/[(2/t-1)+1])=(2/t-1)^2+3(2/t-1)-4
=4/t^2-4/t+1+6/t-3-4
=4/t^2+2/t-6
即f(x)=4/x^2+2/x-6