Chắc bạn ghi nhầm \(x^3\) thành \(n^3\)
\(x^3+y^3+z^3-3xyz=x^3+y^3+3xy\left(x+y\right)-3xy\left(x+y\right)+z^3-3xyz\)
\(=\left(x+y\right)^3+z^3-3xy\left(x+y\right)-3xyz\)
\(=\left(x+y+z\right)\left(\left(x+y\right)^2-z\left(x+y\right)+z^2\right)-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)\left(x^2+y^2+z^2-xy-yz-zx\right)\)