原式=[3-(a+1)(a-1)]/(a+1)÷(a-2)²/(a+1)
=-(a+2)(a-2)/(a+1)×(a+1)/(a-2)²
=-(a+2)/(a-2)为什么[3/(a+1)-a+1] =[3-(a+1)(a-1)]通分
采纳吧