a-2/a²-1÷（a-1-2a-1/a+1)=a-2/a²-1÷（（a²-1-（2a-1））/a+1)=a-2/a²-1÷（（a²-1-2a+1）/a+1)=a-2/a²-1÷（（a²-2a）/a+1)=(a-2)/(a-1)(a+1)* (a+1)/a(a-2)=1/a(a-1)...