y=(1-x^2)/(1+x^2)的值域导数法!)答案为(-1,1]
令f(x)=1-x^2,g(x)=1+x^2，有f'(x)=-2x,g'(x)=2x
y=f(x)/g(x)=(f'g-fg')/g^2=(-2x*(1+x^2)-(1-x^2)*2x)/(1+x^2)^2=(-4x)/(1+x^2)^2