\(\dfrac{y^2-x^2}{x^3-3x^2y+3xy^2-y^3}\\ =-\dfrac{x^2-y^2}{x^3-3x^2y+3xy^2-y^3}\\ =-\dfrac{\left(x+y\right)\left(x-y\right)}{\left(x-y\right)^3}\\ =-\dfrac{x+y}{\left(x-y\right)^2}\\ =-\dfrac{x+y}{x^2-2xy+y^2}\)
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