原式={(m-1)²/[(m-1)(m+1)]}÷[(m-1)(m+1)/(m+1)-(m-1)/(m+1)]
=[(m-1)/(m+1)]÷[m(m-1)/(m+1)]
=[(m-1)/(m+1)]×{(m+1)/[m(m-1)]}
=1/m