:(2/3a^4b^2-1/9a^2b^6)除以(-1/3ab^3)^2
=(2/3a^4b^2-1/9a^2b^6)÷(1/9a^2b^6)
=6a²b^(-4)-1
当a=1/2,b=-1时
原式=3/2-1=1/2