先化简1-a−1a÷（aa+2-1a2+2a），再从±2，±1，0中选取一个适当的数作为a的值代入求值．

Question

先化简1-(a−1)/(a)÷（(a)/(a+2)-(1)/(a2+2a)），再从±2，±1，0中选取一个适当的数作为a的值代入求值．

Answer 1

原式=1-(a−1)/(a)÷[(a2)/(a(a+2))-(1)/(a(a+2))]
=1-(a−1)/(a)÷((a+1)(a−1))/(a(a+2))
=1-(a−1)/(a)•(a(a+2))/((a+1)(a−1))
=1-(a+2)/(a+1)
=(a+1−a−2)/(a+1)
=-(1)/(a+1)；
当a=2时，原式=-(1)/(2+1)=-(1)/(3)．