Câu a : \(x^2-6x+8\)

\(=x^2-2x-4x+8\)

\(=x\left(x-2\right)-4\left(x-2\right)\)

\(=\left(x-2\right)\left(x-4\right)\)

Câu b : \(x^2\left(x-3\right)+12-4x\)

\(=x^2\left(x-3\right)+4\left(3-x\right)\)

\(=x^2\left(x-3\right)-4\left(x-3\right)\)

\(=\left(x-3\right)\left(x^2-4\right)\)

\(=\left(x-3\right)\left(x-2\right)\left(x+2\right)\)

Câu c : \(x^3-\dfrac{1}{4}x\)

\(=x\left(x^2-\dfrac{1}{4}\right)\)

\(=x\left(x-\dfrac{1}{2}\right)\left(x+\dfrac{1}{2}\right)\)

Câu d : \(\left(x+2\right)^2-x^2+4\)

\(=\left(x+2\right)^2-\left(x^2-4\right)\)

\(=\left(x+2\right)^2-\left(x+2\right)\left(x-2\right)\)

\(=\left(x+2\right)\left(x+2-x+2\right)\)

\(=4\left(x+2\right)\)