（1）a-b/3a+3b ,a+b/5a-5b
（a-b）/（3a+3b）=5(a-b)^2/[15(a+b)(a-b)]
(a+b)/(5a-5b)=3(a+b)^2/[15(a+b)(a-b)]
(2)m-n ,mn/n-m
m-n=-(m-n)^2/(n-m)
mn/(n-m)=mn/(n-m)