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x5 + x4 + 1 = x5 - x3 - x2 - x4 + x2 + x + x3 - x - 1
= x2 ( x3 - x - 1 ) - x ( x3 - x - 1 ) + 1 ( x3 - x - 1 )
= ( x3 - x - 1 ) ( x2 - x + 1 )

\(x^8+x^4+1\)
\(=x^8+x^7+x^6-x^7-x^6-x^5+x^5+x^4+x^3-x^3-x^2-x+x^2+x+1\)
\(=\left(x^8+x^7+x^6\right)-\left(x^7+x^6+x^5\right)+\left(x^5+x^4+x^3\right)-\left(x^3+x^2+x\right)+\left(x^2+x+1\right)\)
\(=x^6\left(x^2+x+1\right)-x^5\left(x^2+x+1\right)+x^3\left(x^2+x+1\right)-x\left(x^2+x+1\right)+\left(x^2+x+1\right)\)
\(=\left(x^2+x+1\right)\left(x^6-x^5+x^3-x+1\right)\)
\(x^5-x^4-1\)
\(=x^5-x^4+x^3-x^3+x^2-x-x^2+x-1\)
\(=\left(x^5-x^4+x^3\right)-\left(x^3-x^2+x\right)-\left(x^2-x+1\right)\)
\(=x^3\left(x^2-x+1\right)-x\left(x^2-x+1\right)-\left(x^2-x+1\right)\)
\(=\left(x^2-x+1\right)\left(x^3-x-1\right)\)

\(x^5+x^4+2\)
\(=x^5+x^4+x^2-x^2+1+1\)
\(=\left(x^5-x^2\right)+\left(x^4+x^2+1\right)\)
\(=\left(x^5-x^2\right)+\left(x^4+2x^2-x^2+1\right)+1\)
\(=x^2\left(x^3-1\right)+\left(x^4+2x^2-x^2+1\right)+1\)
\(=x^2\left(x-1\right)\left(x^2+x+1\right)+\left(\left(x^2+1\right)^2-x^2\right)+1\)
\(=x^2\left(x-1\right)\left(x^2+x+1\right)+\left(x^2+1+x\right)\cdot\left(x^2+1-x\right)+1\)
\(=\left(x^3-x^2\right)\left(x^2+x+1\right)+\left(x^2+1+x\right)\cdot\left(x^2+1-x\right)+1\)
\(=\left(x^2+x+1\right)\left(x^3-x^2+x^2+1-x\right)+1\)
\(=\left(x^2+x+1\right)\left(x^3+1-x\right)+1\)

\(x^5+x+1=x^5+x^4-x^4+x^3-x^3+x^2-x^2+x+1\)
\(=\left(x^5+x^4+x^3\right)+\left(x^2+x+1\right)-\left(x^4+x^3+x^2\right)\)
\(=x^3\left(x^2+x+1\right)+\left(x^2+x+1\right)-x^2\left(x^2+x+1\right)\)
\(=\left(x^2+x+1\right)\left(x^3-x^2+1\right)\)
\(x^{10}+x^5+1\)
\(=\left(x^{10}-x^9+x^7-x^6+x^5-x^3+x^2\right)\)
\(+\left(x^9-x^8+x^6-x^5+x^4-x^2+x\right)\)
\(+\left(x^8-x^7+x^5-x^4+x^3-x+1\right)\)
\(=x^2\left(x^8-x^7+x^5-x^4+x^3-x+1\right)\)
\(+x\left(x^8-x^7+x^5-x^4+x^3-x+1\right)\)
\(+\left(x^8-x^7+x^5-x^4+x^3-x+1\right)\)
\(=\left(x^2+x+1\right)\left(x^8-x^7+x^5-x^4+x^3-x+1\right)\)


Đa thức có dạng \(x^{3a+1}+x^{3b+2}+1\) thì đưa về dạng \(\left(x^2+x+1\right)\cdot P\left(x\right)\) bạn nhé!
Bài làm:
\(x^5+x+1\)
\(=\left(x^5-x^2\right)+\left(x^2+x+1\right)\)
\(=x^2\left(x^3-1^3\right)+\left(x^2+x+1\right)\)
\(=x^2\left(x-1\right)\left(x^2+x+1\right)+\left(x^2+x+1\right)\)
\(=\left(x^2+x+1\right)\left(x^3-x^2+1\right)\)
\(x^5+x+1=x^5-x^2+x^2+x+1\)
\(=x^2\left(x^3-1\right)+x^2+x+1\)
\(=x^2\left(x-1\right)\left(x^2+x+1\right)+\left(x^2+x+1\right)\)
\(=\left(x^2+x+1\right)\left(x^2\left(x-1\right)+1\right)\)

<=>x4-x+x2 +x+1= x (x-1) (x2+x+1) + (x2+x+1) = (x2+x+1)(x2-x+1)
chắc có lẽ đúng đó
x5+x4+1=x5+x4+x3+x2+x+1-x3-x2-x
=x3.(x2+x+1)+(x2+x+1)-x.(x2+x+1)
=(x2+x+1)(x3-x+1)