原式=(2a+1)/(a-1)(a+1) ×(a-1)²/[a(a-1)] -1/(a+1)=(2a+1)/[a(a+1)] -a/[a(a+1)]=(2a+1-a)/[a(a+1)]=(a+1)/[a(a+1)]=1/a=1/(-2分之1)=-2
a=-1/2，则a²-(a+1)/a=1/4-1-(-2)=5/4
a²-(a+1)/a=1/4-3=-11/4
