a²/a-1-1/1-a-a-1=a²/(a-1)+1/(a-1)-(a+1)=a²/(a-1)+1/(a-1)-(a²-1)/(a-1)=(a²+1-a²+1)/(a-1)=2/(a-1)当a=1/2时原式=2/(-1/2)=-4希望采纳！
