\(x^4+13x^2+36=x^4+4x^2+9x^2+36\)
\(=x^2\left(x^2+4\right)+9\left(x^2+4\right)=\left(x^2+9\right)\left(x^2+4\right)\)
\(x^4+13x^2+36\)
<=> \(x^4+9x^2+4x^2+36\)
<=> \(x^2\left(x^2+9\right)+4\left(x^2+9\right)\)
<=> \(\left(x^2+9\right)\left(x^2+4\right)\)
\(x^4+13x^2+36\)
\(=x^4+4x^2+9x^2+36\)
\(=\left(x^4+4x^2\right)+\left(9x^2+36\right)\)
\(=x^2\left(x^2+4\right)+9\left(x^2+4\right)\)
\(=\left(x^2+4\right)\left(x^2+9\right)\)
\(x^4+13x^2+36=x^4+4x+9x+36=x\left(x+4\right)+9\left(x+4\right)=\left(x+9\right)\left(x+4\right)\)