a=2x,b=4/(x^2+1)
a^2+b^2>=2ab
当a=b时,a^2+b^2=2ab为最小值
2x=4/(x^2+1)
x^3+x-2=0
(x-1)(x^2+x+2)=0
x=1,当x=1时,
y=4x^2+16/(x^2+1)^2最小值=4+16/4=8