Đặt
\(A=x^4-4x^3+8x+3\)
Giả sử
\(A=\left(x^2+ax+b\right)\left(x^2+cx+d\right)\)
\(=x^4+cx^3+dx^2+ax^3+acx^2+adx+bx^2+bcx+bd\)
\(=x^4+\left(a+c\right)x^3+\left(b+ac+d\right)x^2+\left(ad+bc\right)x+bd\)
\(\left[\begin{array}{nghiempt}a+c=-4\\b+ac+d=0\\ad+bc=8\\bd=3\end{array}\right.\)
\(\left[\begin{array}{nghiempt}a=-2\\b=-3\\c=-2\\d=-1\end{array}\right.\)
\(A=\left(x^2-2x-3\right)\left(x^2-2x-1\right)\)
dài dòng
\(x^4-4x^3+8x+3=x^4-2x^3-2x^3-x^2+4x^2-3x^2+2x+6x+3\)
\(=\left(x^4-2x^3-x^2\right)-\left(2x^3-4x^2-2x\right)-\left(3x^2-6x-3\right)\)
\(=x^2\left(x^2-2x-1\right)-2x\left(x^2-2x-1\right)-3\left(x^2-2x-1\right)\)
\(=\left(x^2-2x-1\right)\left(x^2-2x-3\right)\)
