[1-1/(m-1)]÷(m^2-4m+4)/(m^2-m),其中m=-1
=(m-1-1)/(m-1)÷(m-2)²/m(m-1)
=(m-2)/(m-1)×m(m-1)/(m-2)²
=m/(m-2)
=-1/(-1-2)
=1/3