Áp dụng hằng đẳng thức nâng cao :
\(\left(x+y+z\right)^3=x^3+y^3+z^3+3\left(x+y+z\right)\left(xy+yz+xz\right)-3xyz\)
Vậy \(\left(x+y+z\right)^3-x^3-y^3-z^3=x^3+y^3+z^3+3\left(x+y+z\right)\left(xy+yz+xz\right)-3xyz-x^3-y^3-z^3 =3\left(x+y+z\right)\left(xy+yz+xz\right)-3xyz\)