Answer
$(x+10)^2$
Work Step by Step
The 2 numbers whose product is $ac=
1(100)=100
$ and whose sum is $b=
20
$ are $\left\{
10,10
\right\}.$ Using these two numbers to decompose the middle term of the given trinomial, $
x^2+20x+100
,$ then the factored form is
\begin{array}{l}
x^2+10x+10x+100
\\\\=
(x^2+10x)+(10x+100)
\\\\=
x(x+10)+10(x+10)
\\\\=
(x+10)(x+10)
\\\\=
(x+10)^2
.\end{array}