Ta có:\(x^3+9x^2+11x-21\)
\(=x^3-x^2+10x^2-10x+21x-21=x^2\left(x-1\right)+10x\left(x-1\right)+21\left(x-1\right)\)
\(=\left(x^2+10x+21\right)\left(x-1\right)=\left(x^2+3x+7x+21\right)\left(x-1\right)\)
\(=\left[x\left(x+3\right)+7\left(x+3\right)\right]\left(x-1\right)\)
\(=\left(x+3\right)\left(x+7\right)\left(x-1\right)\)
x^3+9x^2+11x-21=x^3-x^2+10x^2-10x+21x-21=(x^3-x^2)+(10x^2-10x)+(21x-21)
=x^2(x-1)+10x(x-1)+21(x-1)=(x-1)(x^2+10x+21)=(x-1)(x^2+3x+7x+21)=(x-1)[(x^2+3x)+(7x+21)]
=(x-1)(x+7)(x+3)
\(x^3+9x^2+11x-21\)
\(=x^3-x^2+10x^2-10x+21x-21\)
\(=\left(x^3-x^2\right)+\left(10x^2-10x\right)+\left(21x-21\right)\)
\(=x^2\left(x-1\right)+10x\left(x-1\right)+21\left(x-1\right)\)
\(=\left(x^2+10x+21\right)\left(x-1\right)\)
\(=\left(x^2+10x+25-4\right)\left(x-1\right)\)
\(=\left[\left(x-5\right)^2-4\right]\left(x-1\right)\)
\(=\left(x-7\right)\left(x-3\right)\left(x-1\right)\)
ko chắc phần sau lắm nhé
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