Answer
$(x+2)(x-2)(x^2+5)$
Work Step by Step
The 2 numbers whose product is $ac=
1(-20)=-20
$ and whose sum is $b=
1
$ are $\{
-4,5
.\}$ Using these two numbers to decompose the middle term of the given expression, $
x^4+x^2-20
,$ then the factored form is
\begin{array}{l}\require{cancel}
x^4-4x^2+5x^2-20
\\\\=
(x^4-4x^2)+(5x^2-20)
\\\\=
x^2(x^2-4)+5(x^2-4)
\\\\=
(x^2-4)(x^2+5)
.\end{array}
Using $a^2-b^2=(a+b)(a-b)$, then the complete factored form of the equation above is
\begin{array}{l}\require{cancel}
(x+2)(x-2)(x^2+5)
.\end{array}
