(-2mn)^3(mn+1/2)(m^2n^2-1/2mn+1/4)
=(-2mn)^3(mn+1/2)(mn-1/2)^2
=(-2mn)^3(m^2n^2-1/4)(mn-1/2)
=(-2mn)^3[(mn)^3-(mn)^2/2-mn/4+1/8]
=-8(mn)^6+4(mn)^5+2(mn)^4-(mn)^3