[1-1/(a-1)]÷[(a²-4a+4)/(a²-a)]
=(a-2)/(a-1)÷[(a-2)²/a(a-1)]
=(a-2)/(a-1)*[a(a-1)/(a-2)²]
=a/(a-2)
当a=-1时,原式=-1/(-1-2)=1/3