a) Ta có: \(x^2+9x+20\)
\(=x^2+4x+5x+20\)
\(=x\left(x+4\right)+5\left(x+4\right)\)
\(=\left(x+4\right)\left(x+5\right)\)
b) Ta có: \(x^2+x-12\)
\(=x^2+4x-3x-12\)
\(=x\left(x+4\right)-3\left(x+4\right)\)
\(=\left(x+4\right)\left(x-3\right)\)
c) Ta có: \(6x^2-11x-16\)
\(=6\left(x^2-\frac{11}{6}x-\frac{16}{6}\right)\)
\(=6\left(x^2-2\cdot x\cdot\frac{11}{12}+\frac{121}{144}-\frac{505}{144}\right)\)
\(=6\left[\left(x-\frac{11}{12}\right)^2-\frac{505}{144}\right]\)
\(=6\left(x-\frac{11+\sqrt{505}}{12}\right)\left(x-\frac{11-\sqrt{505}}{12}\right)\)
d) Ta có: \(4x^2-8x-5\)
\(=4x^2-10x+2x-5\)
\(=2x\left(2x-5\right)+\left(2x-5\right)\)
\(=\left(2x-5\right)\left(2x+1\right)\)
e) Ta có: \(x^3-6x^2-x+30\)
\(=x^3+2x^2-8x^2-16x+15x+30\)
\(=x^2\left(x+2\right)-8x\left(x+2\right)+15\left(x+2\right)\)
\(=\left(x+2\right)\left(x^2-8x+15\right)\)
\(=\left(x+2\right)\left(x^2-3x-5x+15\right)\)
\(=\left(x+2\right)\left[x\left(x-3\right)-5\left(x-3\right)\right]\)
\(=\left(x+2\right)\left(x-3\right)\left(x-5\right)\)
g) Ta có: \(x^3+9x^2+23x+15\)
\(=x^3+x^2+8x^2+8x+15x+15\)
\(=x^2\left(x+1\right)+8x\left(x+1\right)+15\left(x+1\right)\)
\(=\left(x+1\right)\left(x^2+8x+15\right)\)
\(=\left(x+1\right)\left(x^2+3x+5x+15\right)\)
\(=\left(x+1\right)\left[x\left(x+3\right)+5\left(x+3\right)\right]\)
\(=\left(x+1\right)\left(x+3\right)\left(x+5\right)\)
h) Ta có: \(2x^4-x^3-9x^2+13x\)
\(=x\left(2x^3-x^2-9x+13\right)\)
i) Ta có: \(x^4+2x^3-16x^2-2x+15\)
\(=x^4-3x^3+5x^3-15x^2-x^2+3x-5x+15\)
\(=x^3\left(x-3\right)+5x^2\left(x-3\right)-x\left(x-3\right)-5\left(x-3\right)\)
\(=\left(x-3\right)\left(x^3+5x^2-x-5\right)\)
\(=\left(x-3\right)\left[x^2\left(x+5\right)-\left(x+5\right)\right]\)
\(=\left(x-3\right)\left(x+5\right)\left(x^2-1\right)\)
\(=\left(x-3\right)\left(x+5\right)\left(x-1\right)\left(x+1\right)\)