# What is the domain of f(x)= (x^2+18x+18 ) / ( x^2+9x+20)?

$x \in \mathbb{R}$
Defined and continuous $\iff$ $x \in \mathbb{R}$
We can plug in any value for $x$ and get a value for $f \left(x\right)$. It doesn't matter that it's a fraction- even if $x$ is zero, we get a value, $\frac{9}{10}$.