\(=\left(x+4\right)^2-1=\left(x+4-1\right)\left(x+4+1\right)=\left(x+3\right)\left(x+5\right)\)
\(x^2+8x+15=x\left(x+3\right)+5\left(x+3\right)=\left(x+3\right)\left(x+5\right)\)
\(=x^2+3x+5x+15=\left(x+3\right)\left(x+5\right)\)
Tách 8x=5x+3x rồi lấy nhân tử chung rồi bạn tự làm nhé~