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