X4 +3x³+x²-12x-20 thành nhân tử
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Ta có: \(\left(4x+1\right)\left(12x-1\right)\left(3x+2\right)\left(x+1\right)-4\)
\(=\left(12x^2+8x+3x+2\right)\left(12x^2+12x-x-1\right)-4\)
\(=\left(12x^2+11x+2\right)\left(12x^2+11x-1\right)-4\)
\(=\left(12x^2+11x\right)^2+\left(12x^2+11x\right)-6\)
\(=\left(12x^2+11x+3\right)\left(12x^2+11x-2\right)\)
\(=\left(x^4+x^3+x^2\right)+\left(x^2+x+1\right)\)
\(=x^2\left(x^2+x+1\right)+\left(x^2+x+1\right)\)
\(=\left(x^2+1\right)\left(x^2+x+1\right)\)
\(x^4+x^3+2x^2+x+1\)
\(=x^4+x^3+x^2+x^2+x+1\)
\(=\left(x^2+x+1\right)\left(x^2+1\right)\)
\(x^5-x^4-30x^3=x^3\left(x^2-x-30\right)=x^3\left(x-6\right)\left(x+5\right)\)
\(=\left(x^6+2x^5+x^4\right)-2\left(x^5+2x^4+x^3\right)+2\left(x^4+2x^3+x^2\right)\)
\(=x^2\left(x^2+x\right)^2-2x\left(x^2+x\right)^2+2\left(x^2+x\right)^2\)
\(=\left(x^2+x\right)^2\left(x^2-2x+2\right)\)
\(=x^2\left(x+1\right)^2\left(x^2-2x+2\right)\)
\(x^4+6x^3+11x^2+6x+1\)
\(=x^4+3x^3+x^2+3x^3+9x^2+3x+x^2+3x+1\)
\(=\left(x^2+3x+1\right)^2\)
\(x^4+6x^3+7x^2-6x+1\)
\(=x^4-2x^2+1+6x^3+9x^2-6x\)
\(=\left(x^2-1\right)^2+6x\left(x^2-1\right)+9x^2\)
\(=\left(x^2+3x-1\right)^2\)
\(x^3-12x^2-12x+1=x^3+x^2-13x^2-13x+x+1=x^2\left(x+1\right)-13x\left(x+1\right)+x+1=\left(x+1\right)\left(x^2-13x+1\right)\)
\(=\left(x^3+1\right)-12x\left(x+1\right)\)
= \(\left(x+1\right)\left(x^2-x+1\right)-12x\left(x+1\right)\)
= \(\left(x+1\right)\left(x^2-13x+1\right)\)
= \(\left(x+1\right)\left(x-\frac{13+\sqrt{165}}{2}\right)\left(x-\frac{13-\sqrt{165}}{2}\right)\)
\(\left(x^2+6x-1\right)^2+2x^2+x^4+2\left(x^2+6x-1\right)\left(x^2+1\right)\)
\(\left(x^2+6x-1\right)^2+2\left(x^2+6x-1\right)\left(x^2+1\right)+\left(x^2+1\right)^2-1=\left(x^2+6x-1+x^2+1\right)^2-1=\left(2x^2+6x\right)^2-1=\left(2x^2+6x-1\right)\left(2x^2+6x+1\right)\)
\(\left(x^2+6x-1\right)^2+2\left(x^2+6x-1\right)\left(x^2+1\right)+x^4+2x^2\)
\(=\left(x^2+6x-1\right)\left(x^2+6x-1+2x^2+2\right)+x^4+2x^2\)
\(=\left(x^2+6x-1\right)\left(3x^2+6x+1\right)+x^4+2x^2\)
\(=\left(2x^2+6x-1\right)\left(2x^2+6x+1\right)\)
\(=x^4+2x^3+x^3+2x^2-x^2-2x-10x-20=x^3\left(x+2\right)+x^2\left(x+2\right)-x\left(x+2\right)-10\left(x-2\right)\)
\(=\left(x+2\right)\left(x^3+x^2-x-10\right)=\left(x+2\right)\left(x-2\right)\left(x^2+3x+5\right)\)
\(x^4+3x^3+x^2-12x-20\)
\(=x^4+3x^3+5x^2-4x^2-12x-20\)
\(=x^2\left(x^2+3x+5\right)-4\left(x^2+3x+5\right)\)
\(=\left(x^2-4\right)\left(x^2+3x+5\right)\)
\(=\left(x-2\right)\left(x+2\right)\left(x^2+3x+5\right)\)