原式=[a²/(a+1)-(a-1)]×(a²-1)/a
=[a²-(a+1)(a-1)]/(a+1)×(a+1)(a-1)/a
=(a²-a²+1)/(a+1)×(a+1)(a-1)/a
=1/(a+1)×(a+1)(a-1)/a
=(a-1)/a