(-2ab)^3(5a^2b - 1/2 ab^2 + 1/4 b^3)
= -8a^3b^3(5a^2b - 1/2 ab^2 + 1/4 b^3)
= (-8a^3b^3)× (5a^2b) - ( -8a^3b^3)×(1/2 ab^2) + (-8)a^3b^3) ×(1/4b^3)
= - 40 a^5b^4 + 4a^4b^5 - 2a^3b^6