(m+2分之2m-m-2分之m)除以m的平方-4分之m
=m*(2/(m+2) - 1/(m-2) )*(m^2-4)/m
=(2/(m+2) - 1/(m-2) )*(m+2)(m-2)
=2(m-2) -(m+2)
=2m-4 -m-2
=m-6