原式=a²(a²-a+1)/(a+1)(a²-a+1)-a²(a²+a+1)/(a-1)(a²+a+1)=a²/(a+1)-a²/(a-1)=a²[1/(a+1)-1/(a-1)]=a²[(a-1-a-1)/(a+1)(a-1)]=-2a²/(a²-1)