f(x)=1/(1/x-1)^2=x^2/(1-x)^2f'(x)=(2x(1-x)^2+2(1-x)x^2)/(1-x)^4=(2x)/(1-x)^3
因为f（x）=x^2/(1-x)^2.所以f（x）导数为[2x(1-x)^2-x^2*2(-1)(1-x)]/ (1-x)^4.
