\(x^4 + 6x^3 + 7x^2 -6x +1\)
\(=x^2.\left(x^2+6x+7-\dfrac{6}{x}+\dfrac{1}{x^2}\right)\)
\(=x^2\left[\left(x^2+\dfrac{1}{x^2}\right)+6.\left(x-\dfrac{1}{x}\right)+7\right]\)
Đặt \(x-\dfrac{1}{x}=y\).
\(=x^2\left(y^2+2+6y+7\right)\)
\(=x^2\left(y+3\right)^2\)
\(=\left(xy+3x\right)^2\)
\(=\left[x\left(x-\dfrac{1}{x}\right)+3x\right]^2\)
\(=\left(x^2+3x-1\right)^2\)