f'(x)=[4(x^2+1)-4x*2x]/(x^2+1)^2
=4(1-x^2)/(x^2+1)^2
f'(x)=0解得x=±1
f(-1)=-2,f(1)=2,f(-2)= -8/5,f(2)=8/5
所以最大值2,最小值 -2