đặt m=3k+r,n=3s+t(r,s,<3)
$\begin{array}{l}
{x^m} + {x^n} + 1 = {x^{3k + r}} + {x^{3s + t}} + 1 = {x^{3k}}.{x^r} - {x^r} + {x^{3s}}.{x^t} - {x^t} + {x^r} + {x^t} + 1\\
= {x^r}\left( {{x^{3k}} - 1} \right) + {x^t}\left( {{x^{3s}} - 1} \right) + {x^r} + {x^t} + 1\\
\left( {{x^{3k}} - 1} \right)...