计算(a/5-b/2)(a^2/25+ab/10+b^2/4)
问题描述：
计算(a/5-b/2)(a^2/25+ab/10+b^2/4)
答
设A=a/5,B=b/2
(a/5-b/2)(a^2/25+ab/10+b^2/4)
=(A-B)(A^2+AB+B^2)
=A^3-B^3
=a^3/125-b^3/8