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roblox20117

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

$=x(x^3+x^2y+xy^2+y^3)-y\left(x^3+x^2y+xy^2+y^3)\right.$

$=x^4+x^3y+x^2y^2+x^2y^3+xy^3-x^3y-x^2y^3-xy^3-y^4$

$ = x^4 - y^4 + (x^3y - x^3y) + (x^2y^2 - x^2y^2) + (x^2y^3 - x^2y^3)$

$ = x^4 - y^4 (đpcm)$

$ (x-y-z)^2$

$=\left(x-y\right)^2-2\left(x-y\right)z+z^2$

$ = x^2 - 2xy + y^2 - (2x + 2y)z + z^2$

$ = x^2 - 2xy + y^2 - 2xz + 2yz+ z^2$

$ = x^2 + y^2 + z^2 - 2xy + 2yz - 2xz (đpcm)$