#### Answer

Constant should be added to the binomial: $100$
Trinomial: $x^{2}+20x+100$
Factors: $(x+10)(x+10)$

#### Work Step by Step

$x^{2}+20x$
By adding the square of half the co-efficient of $x$, we get
$x^{2}+20x+100$
By factoring,
$x^{2}+20x +100=(x+10)^{2}$
$(x+10)(x+10)$