\(x^4-3x^2+9\)
\(=\left(x^2\right)^2+2.x^2.3+3^2-9x^2\)
\(=\left(x^2+3\right)^2-\left(3x\right)^2\)
\(=\left(x^2-3x+3\right)\left(x^2+3x+3\right)\)
\(x^4+3x^2+4\)
\(=\left(x^2\right)^2+2.x^2.2+2^2-x^2\)
\(=\left(x^2+2\right)^2-x^2\)
\(=\left(x^2-x+2\right)\left(x^2+x+2\right)\)