先化简,再求值（3/4a^3+1/2a^3b^3-1/9a^2b^4）/(-1/3ab)^2,其中a=1/2,b=-4 清楚一点

（3/4a^3+1/2a^3b^3-1/9a^2b^4）/(-1/3ab)^2
=(a^2/36)（27a+18ab^3-4b^4）/(1/9a^2b^2)
=(27a+18ab^3-4b^4）/4b^2
=27a/4b^2+9ab/2-b^2
=27*(1/2)/4*(-4)^2+9*(1/2)*4/2-(-4)^2
=27/128-7
=-869/128