\(\frac{x^2+y^2}{\left(x-y\right)^3}-\frac{2xy}{\left(x-y\right)^3}=\frac{x^2-2xy+y^2}{\left(x-y\right)^3}=\frac{\left(x-y\right)^2}{\left(x-y\right)^3}=x-y\)
\(\frac{x^2+y^2}{\left(x-y\right)^3}-\frac{2xy}{\left(x-y\right)^3}\\ =\frac{x^2+y^2-2xy}{\left(x-y\right)^3}\\ =\frac{\left(x-y\right)^2}{\left(x-y\right)^3}\\ =\frac{1}{x-y}\)