x^4+ax^2+b chia hết cho x^2+x+1
Đặt
x4+ax2+b=(x2+x+1)(x2+cx+d)
⇔x4+ax2+b=x4+(c+1)x3+(c+d+1)x2+(c+d)x+d
$\Leftrightarrow \left\{\begin{matrix} c+1=0 \\ c+d+1=a \\ c+d=0 \\ d=b \end{matrix} \right.
\Leftrightarrow \left\{\begin{matrix} a=b=d=1 \\ c=-1 \end{matrix} \right.$