（2011•江宁区一模）先化简：（3a−1-1a+1）÷aa2−1，然后选取一个适当的a值代入求值．

Question

（2011•江宁区一模）先化简：（(3)/(a−1)-(1)/(a+1)）÷(a)/(a2−1)，然后选取一个适当的a值代入求值．

Answer 1

原式=[(3(a+1))/((a+1)(a−1))−(a−1)/((a+1)(a−1))]•(a2−1)/(a)
=(3a+3−a+1)/((a+1)(a−1))•((a+1)(a−1))/(a)
=(2a+4)/(a)．
由题意得a≠±1，∴取当a=2时，代入得；
=(2×2+4)/(2)=4．