解原式
=（m-2+1)/(m-2)×（2m-4）/(m^2-1)
=（m-1)/(m-2)×2（m-2）/(m^2-1)
=2（m-1)/(m^2-1)
=2(m-1)/(m-1)(m+1)
=2/(m+1)
=2/(-5+1)
=-1/2