Answer
$(x^2+5)(x+2)(x-2)$
Work Step by Step
Given: $x^4+x^2-20 \\ $
$1.)$ Let: $u=x^2 $
$u^2+u-20$
$2.)$ Factor $u^2+u-20$
$(u+5)(u-4)$
$3.)$ Substitute $u=x^2$
$(x^2+5)(x^2-4)$
$4.)$ Factor $(x^2-4)$
$(x^2+5)(x+2)(x-2)$
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