\(=\text{[}\left(x+3\right)\left(x+6\right)\text{]}\text{[}\left(x+4\right)\left(x+5\right)\text{]}+1=\left(x^2+9x+18\right)\left(x^2+9x+20\right)+1.\text{Đ}\text{ạt}:x^2+9x+18=a\Rightarrow\left(x+3\right)\left(x+4\right)\left(x+5\right)\left(x+6\right)+1=a\left(a+2\right)+1=a^2+2a+1=\left(a+1\right)^2=\left(x^2+9x+19\right)^2\)