1)(x-y)/[(x-y)(x²+xy+y²)-(x+y)/[(x+y)(x²-xy+y²)]=1/(x²+xy+y²)+1/(x²-xy+y²)=
=(x²-xy+y²-x²-xy-y²)/[(x²+xy+y²)(x²-xy+y²)]=-2xy/[(x²+xy+y²)(x²-xy+y²)]
2)(x-y)+xy/(x-y)=(x²-2xy+y²+xy)/(x-y)=(x²-xy+y²)/(x-y)
3)(x+y)-xy/(x+y)=(x²+2xy+y²-xy)/(x+y)=(x²+xy+y²)/(x+y)
4)-2xy/[(x²+xy+y²)(x²-xy+y²)] *(x²-xy+y²)/(x-y)*(x²+xy+y²)/(x+y)=
=-2xy/(x²-y²)=2xy/(y²-x²)