答：
(-2a)^2(1/2ab+b^2)+(3a^2-2ab^2)(-1/2a)
=4a^2(1/2ab+b^2)-1/2a^2(3a-2b^2)
=a^2(2ab+4b^2-3/2a+b^2)
=a^2(2ab+5b^2-3/2a)
=2a^3b+5(ab)^2-3/2a^3