y=4(x^2+1)+16/(x^2+1)^2-4=2(x^2+1)+2(x^2+1)+16/(x^2+1)^2-4
>=3倍3次根号[2(x^2+1)*2(x^2+1)*16/(x^2+1)^2]-4=3*4-4=8,所以,最小值为8.