Phân tích đa thức thành nhân tử:
6x4+23x3+13x2_23x+7
10x4+19x3-21x2-5x+2
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\(5x^2-5x-10=5x^2+5x-10x-10=5x\left(x+1\right)-10\left(x+1\right)=\left(5x-10\right)\left(x+1\right)\)
5x2−5x−10=5x2+5x−10x−10=5x(x+1)−10(x+1)=(5x−10)(x+1)
\(x^3+5x^2+5x+1\)
\(=\left(x+1\right)\left(x^2+x+1\right)+5x\left(x+1\right)\)
\(=\left(x+1\right)\left(x^2+6x+1\right)\)
\(=x^2+2\cdot\dfrac{5}{2}x+\dfrac{25}{4}-\dfrac{25}{4}-2\\ =\left(x+\dfrac{5}{2}\right)^2-\dfrac{33}{4}\\ =\left(x+\dfrac{5}{2}-\dfrac{\sqrt{33}}{2}\right)\left(x+\dfrac{5}{2}+\dfrac{\sqrt{33}}{2}\right)\)
\(\left(x^2+5x\right)^2-2\left(x^2+5x\right)-24\\ =\left[\left(x^2+5x\right)^2-6\left(x^2+5x\right)\right]+\left[4\left(x^2+5x\right)-24\right]\\ =\left(x^2+5x\right)\left(x^2+5x-6\right)+4\left(x^2+5x-6\right)\\ =\left(x^2+5x-6\right)\left(x^2+5x+4\right)\\ =\left(x^2-x+6x-6\right)\left(x^2+4x+x+4\right)\\ =\left[x\left(x-1\right)+6\left(x-1\right)\right]+\left[x\left(x+4\right)+\left(x+4\right)\right]\\ =\left(x-1\right)\left(x+1\right)\left(x+4\right)\left(x+6\right)\)
#)Giải :
\(x^3-2x-4\)
\(=x^3+2x^2-2x^2+2x-4x-4\)
\(=x^3+2x^2+2x-2x^2-4x-4\)
\(=x\left(x^2+2x+2\right)-2\left(x^2+2x+2\right)\)
\(=\left(x-2\right)\left(x^2+2x+2\right)\)
\(x^4+2x^3+5x^2+4x-12\)
\(=x^4+x^3+6x^2+x^3+x^2+6x-2x^2-2x-12\)
\(=x^2\left(x^2+x+6\right)+x\left(x^2+x+6\right)-2\left(x^2+x+6\right)\)
\(=\left(x^2+x+6\right)\left(x^2+x-2\right)\)
\(=\left(x^2+x+6\right)\left(x-1\right)\left(x+2\right)\)
Câu 1.
Đoán được nghiệm là 2.Ta giải như sau:
\(x^3-2x-4\)
\(=x^3-2x^2+2x^2-4x+2x-4\)
\(=x^2\left(x-2\right)+2x\left(x-2\right)+2\left(x-2\right)\)
\(=\left(x-2\right)\left(x^2+2x+2\right)\)
\(=x^2-5x+\dfrac{25}{4}-\dfrac{29}{4}\)
\(=\left(x-\dfrac{5}{2}\right)^2-\dfrac{29}{4}\)
\(=\left(x-\dfrac{5}{2}-\dfrac{\sqrt{29}}{2}\right)\left(x-\dfrac{5}{2}+\dfrac{\sqrt{29}}{2}\right)\)
\(=2x^2-6x+x-3=\left(x-3\right)\left(2x+1\right)\)
\(=2x^2-6x+x-3=2x\left(x-3\right)+\left(x-3\right)=\left(2x+1\right)\left(x-3\right)\)