f(x)=√(2/3x^2-4x+9)
=√[(2x^2-12x+27)/3]
=√[(2(x^2-6x+9)+9)/3]
=√[(2(x-3)^2+9)/3]
当x=3时,f(x)min=√3
所以,f(x)∈(√3,+∝)