b/[a-b]+b的立方/【a的立方-2a方b+ab方】+【ab+b方】/【b方-a方】
=b/(a-b)+b³/[a(a-b)²]+b(a+b)/[(b-a)(b+a)]
=b/(a-b)+b³/[a(a-b)²]-b/(a-b)
=b³/[a(a-b)²]