原式=[(a+1)/(a+1)(a-1)-(a-1)/(a+1)(a-1)-(a²-1)/(a+1)(a-1)]×(a+1)(a-1)
=(a+1-a+1-a²+1)/(a+1)(a-1)/(a+1)(a-1)
=a+1-a+1-a²+1
=-a²+3