再求值

题目

再求值
(1-x)/(x+2)²÷[x-2+3/（x+2)],其中x=1/2
答案是-4/15，

答案

原式=(1-x)/(x+2)²÷{[(x-2)(x+2)+3]/（x+2)}=(1-x)/(x+2)²÷[(x²-4+3)/（x+2)]=(1-x)/(x+2)²÷[(x²-1)/（x+2)]=(1-x)/(x+2)²×[(x+2)/(x²-1)]=(1-x)(x+2)/[(x+2)²(x²...