(1)x^2+6x+9/x^2-9
=(x+3/x)^2-9
=(x+3/x+3)(x+3/x-3)
(2)m^2-3m+2/m^-m
=m^2-4m+2/m^2
通分：
(1)x/6ab^2,y/9a^2bc
3xac/18a^2b^2c,2yb/18a^2b^2c
(2)1/m-n,2/n^2-m^2,3n/2(m+n)
2(n+m)/2(n+m)(n-m),4/2(n+m)(n-m),3n(n-m)/2(n+m)(n-m)