（2+1/a-1-1/a+1）÷（a-a/1-a²）
=(2a²-2+a+1-a+1)/(a+1)(a-1)÷[-(a-a³-a)/(a+1)(a-1)]
=2a²/(a+1)(a-1)×(a+1)(a-1)/a³
=2/a
=2/2
=1