[\frac{b*(b^3)^2:b^3}{b^8:b^5}]^{-1}
[\frac{b*(b^3)^2:b^3}{b^8:b^5}]^{-1}=
=[\frac{b*b^6:b^3}{b^3}]^{-1}=[\frac{b^7:b^3}{b^3}]^{-1}=
=[\frac{b^4}{b^3}]^{-1}=b^{-1}=\frac{1}{b}, b\neq0