(x-y)^3 -3(x-y)^2x+3(x-y)x^2-x^3

`(x-y)^3 -3(x-y)^2 x+3(x-y)x^2 -x^3`

`=(x-y)^3 -3.(x-y)^2 .x +3.(x-y).x^2 -x^3`

`=(x-y-x)^3 =(-y)^3 =-y^3`

$\bullet$ Hằng đẳng thức : `(a-b)^3 =a^3 -3a^2 b+3ab^2 -b^3`