[a^(1/3)-b^(1/3)]^3=a-b+3(ab)^(1/3)(a^(1/3-b^(1/3))[(a-b)^(1/3)]^3=(a-b)[a^(1/3)-b^(1/3)]^3-[(a-b)^(1/3)]^3=3(ab)^(1/3)(a^(1/3-b^(1/3)) （1）因ab>0 所以当a>b>0,或0(a-b)^(1/3)当b>a>0,或0