题目

分数指数幂化简
一、{a+b/[a^(2/3)-a^(1/3)×b^(1/3)+b^(2/3)]}+{a-b/[a^(2/3)+a^(1/3)×b^(1/3)+b^(2/3)]}
二、已知a^(1/2)+a^(-1/2)=3,求[a^(3/2)+a^(-3/2)+1] / [a^2+a^(-2)-2]的值

提问时间：2021-02-11

答案

解一题 运用立方和公式 x³+y³=(x+y)(x²-xy+y²) ；立方差公式x³-y³=(x-y)(x²+xy+y²)
原式={[a^(1/3)+b^(1/3)][a^(2/3)-a^(1/3)×b^(1/3)+b^(2/3)]}/{[a^(2/3)-a^(1/3)×b^(1/3)+b^(2/3)]}+
{[a^(1/3)-b^(1/3)][a^(2/3)+a^(1/3)×b^(1/3)+b^(2/3)]}/{[a^(2/3)+a^(1/3)×b^(1/3)+b^(2/3)]}
=a^(1/3)+b^(1/3)+a^(1/3)-b^(1/3)
=2a^(1/3)
解二题
原式={[a^(1/2)]³+[a^(-1/2)]³+1}/[a²+a^(-2)-2]
={[a^(1/2)+a^(-1/2)][a-a^(1/2)×a^(-1/2)+a^(-1)]+1}/{[a+a^(-1)]²-4}
={3×[a+a^(-1)-1]+1}/﹛{[a^(1/2)+a^(-1/2)]²-2}²-4﹜
=﹛3×{[a^(1/2)+a^(-1/2)]²-3}+1﹜/{[(3²-2)]²-4}
=[3×(3²-3)+1]/(7²-4)
=(3×6+1)/45
=19/45