已知x∈R,求y=x^2+2/根号x^2+1=x^2+1/根号x^2+ 1/根号x^2+ 1
>=3{3次根下[x^2 *(1/根号x^2)*(1/根号x^2)]}+1=4
x=1or -1时取=,有最小值为4