（1） x+y+2y^2/(x-y)
=[(x+y)(x-y)+2y^2]/(x-y)
=(x^2+y^2)/(x-y)
(2) (3a^2/a+b)^3(a^2-b^2)除以（b-a/a+b）^2
=(27a^6)/(a+b)^3*(a+b)(a-b)*(a+b)^2/(a-b)^2
=27a^6/(a-b)