b.
\(x^4-4x^2+12x-9=0\)
\(\Leftrightarrow x^4-x^3+x^3-x^2-3x^2+3x+9x-9=0\)
\(\Leftrightarrow\left(x^4-x^3\right)+\left(x^3-x^2\right)-\left(3x^2-3x\right)+\left(9x-9\right)=0\)
\(\Leftrightarrow x^3\left(x-1\right)+x^2\left(x-1\right)-3x\left(x-1\right)+9\left(x-1\right)=0\)
\(\Leftrightarrow\left(x-1\right)\left(x^3+x^2-3x+9\right)=0\)
\(\Leftrightarrow\left(x-1\right)\left(x^3+3x^2-2x^2-6x+3x+9\right)=0\)
\(\Leftrightarrow\left(x-1\right)\left(x+3\right)\left(x^2-2x+3\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x-1=0\\x+3=0\\x^2-2x+3=0\left(loại\right)\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=1\\x=-3\end{matrix}\right.\)
Vậy.............
