\(=\left(x^2+8x+15\right)\left(x^2+8x+7\right)+15\)
đặt:\(^{x^2+8x+11=t}\)
ta co \(\left(t+4\right)\left(t-4\right)+15=t^2-16+15=t^2-1\)
\(=\left(t-1\right)\left(t+1\right)\Rightarrow\left(x^2+8x+11-1\right)\left(x^2+8x+11+1\right)\)
\(\Rightarrow\left(x^2+8x+12\right)\left(x^2+8x+10\right)\)