Question

How to factorise x^4+5x^2+9 ?

Answer 1

let $x^{2} =y$
$y^{2} +5y+9=0$

apply quadratic formual...
I think you equation is
$x^{4} +6 x^{2} +9=0$
then,
$y^{2}+6y+9=0$
$y^{2}+3y+3y+9=0$
 $(y+3)(y+3)=0$
 $y=-3,-3$
 $x=i \sqrt{3} & i \sqrt{3}$
(Note: i is iota)