# Domain and range stuff

#### pinkpixie

Hi! Doing a course in pre-cal and I messed up on a question. Could someone tell me why f(x) = √x + 4/x - 4 has a domain of [-4, 4] U [4, infinity)

Thanks!

#### romsek

Math Team
do you mean $\sqrt{\dfrac{x+4}{x-4}}$ ?

remember that $\sqrt{x}$ is only defined for $x \geq 0$

• pinkpixie

#### pinkpixie

do you mean $\sqrt{\dfrac{x+4}{x-4}}$ ?

remember that $\sqrt{x}$ is only defined for $x \geq 0$
yes! sorry, I'm new and haven't really figured out the codes when typing in math equations

#### romsek

Math Team
just as a double check, we want $y = \dfrac{x+4}{x-4} \geq 0$

There are 2 suspect points, where things go to 0, $x=-4, ~x=4$

So we split the real line up into $(-\infty, -4) \cup [-4] \cup (-4, 4) \cup  \cup (4, \infty)$

we can make a table of things

$\begin{matrix} \text{Interval}&\text{Quotient}&\text{Result}\\ (-\infty, -4) &\dfrac N N &P\\ [-4] &\dfrac 0 N&0\\ (-4,4)&\dfrac P N &N\\  &\dfrac P 0 &\text{not defined}\\ (4, \infty)&\dfrac P P &P \end{matrix}$

and we see that the legit domain is

$(-\infty, -4] \cup (4, \infty)$

Which incidentally doesn't match your original post.

• idontknow