原式=[(a+b)^3/(8a^3b^6)*(1/4)*(a-b)^2]/{[(a+b)^2(a-b)^2]/(a^2b^4)}
=[(a+b)/8a^3b^6]*a^2b^4*(1/4)
=1/32(a+b)/(ab^2)
=1/32×(-1/2+2/3)/[(-1/2)*(4/9)]
=-3/128