[(a4/3-8a1/3b) / (4b2/3+2三次根号ab+a2/3) ] /(1-2三次根号b/a)乘以三
a3-b3=(a-b)3-(-3a2b+3ab2)
=(a-b)(a-b)2+3ab(a-b)
=(a-b)(a2-2ab+b2+3ab)=(a-b)(a2+ab+b2)