ccccccccccccccc Thêm bớt để xuất hiện nhân từ chung 3 x mũ 2 - 5xy - 8 y mũ 2 x mũ 3 -7x -6 x mũ 5 + x mũ 4 + 1 x mũ 4 - x mũ 3 -x -1 x mũ 2 - 5x +6

$\displaystyle  \begin{array}{{>{\displaystyle}l}}
a.\\
3x^{2} -5xy-8y^{2}\\
=3x^{2} -3y^{2} -5xy-5y^{2}\\
=3\left( x^{2} -y^{2}\right) -5y( x+y)\\
=3( x-y)( x+y) -5y( x+y)\\
=( x+y)( 3x-3y-5y)\\
=( x+y)( 3x-8y)\\
b.\\
x^{3} -7x-6\\
=x^{3} -x-6x-6\\
=x\left( x^{2} -1\right) -6( x+1)\\
=x( x-1)( x+1) -6( x+1)\\
=( x+1)\left( x^{2} -x-6\right)\\
c.\\
x^{5} +x^{4} +1\\
=x^{5} -x^{3} +x^{2} +x^{4} -x^{2} +x+x^{3} -x+1\\
=x^{2}\left( x^{3} -x+1\right) +x\left( x^{3} -x+1\right) +x^{3} -x+1\\
=\left( x^{3} -x+1\right)\left( x^{2} +x+1\right)\\
d.\\
x^{4} -x^{3} -x-1\\
=x^{4} -x^{3} -x^{2} +x^{2} -x-1\\
=x^{2}\left( x^{2} -x-1\right) +x^{2} -x-1\\
=\left( x^{2} -x-1\right)\left( x^{2} +1\right)\\
e.\\
x^{2} -5x+6\\
=x^{2} -3x-2x+6\\
=x( x-3) -2( x-3)\\
=( x-3)( x-2)
\end{array}$