题目好像有错
f(x)=-4(x^2-1)/(x^2+1)^2f'(x)=-4[2x(x^2+1)^2-(x^2-1)*2(x^2+1)*2x]/(x^2+1)^4 =-4[2x(x^2+1)^2-4x(x^4-1)]/(x^2+1)^4
