（2+a-1分之1-a+1分之1）除以（a-1-a平方分之a)
=[2+1/(a-1)-1/(a+1)]÷a[1-1/(1-a²)]
=[2+1/(a-1)-1/(a+1)]÷a÷[(1-a²)/(1-a²)-1/(1-a²)]
=[2+1/(a-1)-1/(a+1)]÷a÷[(-a²)/(1-a²)]
=[2+1/(a-1)-1/(a+1)]÷a÷a²/(a²-1)
=[2+1/(a-1)-1/(a+1)]÷a³×(a²-1)
=[2(a²-1)+(a+1)-(a-1)]÷a³
=(2a²-2+a+1-a+1)÷a³
=2a²÷a³
=2/a