\(\left(x-y\right)^3+\left(x-y\right)^2+\dfrac{1}{3}\left(x-y\right)+\dfrac{1}{27}\)
\(=\left(x-y\right)^3+3\cdot\left(x-y\right)^2\cdot\dfrac{1}{3}+3\cdot\left(x-y\right)\cdot\left(\dfrac{1}{3}\right)^2+\left(\dfrac{1}{3}\right)^3\)
\(=\left(x-y+\dfrac{1}{3}\right)^3\)