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[(x+y)^3-(x^3+y^3)]/3(x-y)=[(x+y)(x^2+2xy+y^2)-(x+y)(x^2-xy+y^2)]/3(x-y)=(x+y)(3xy)/3(x-y)=xy(x+y)/(x-y)
(x+y)xy/(x-y)