Đặt \(x^2-2x+4=a\)
Khi đó \(\left(x^2-2x+3\right)\left(x^2-2x+5\right)-8=\left(a-1\right)\left(a+1\right)-8\)
\(=a^2-1-8\)
\(=a^2-9\)
\(=\left(a-3\right)\left(a+3\right)\)
\(=\left(x^2-2x+4-3\right)\left(x^2-2x+4+3\right)\)
\(=\left(x^2-2x+1\right)\left(x^2-2x+7\right)\)
\(=\left(x-1\right)^2\left(x^2-2x+7\right)\)