原式=[√(x/4)-2x√(1/x)]÷3√x
=√(x/4)÷3√x-2x√(1/x)÷3√x
=(1÷3)×√(x/4÷x)-(2x÷3)×√(1/x÷x)
=1/3×√(1/4)-(2x/3)×√(1/x²)
=1/3×1/2-(2x/3)×1/x
=1/6-2/3
=-1/2