f(x) = x / sqrt(x+4)

x + 4 = 0

x = -4

OK, so the domain can't include -4 BUT why is it all real numbers greater than -4?

Answer: Domain is all real numbers greater than -4

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- Nov 7th 2008, 09:42 AMmwokFinding domain of this function, don't understand it
f(x) = x / sqrt(x+4)

x + 4 = 0

x = -4

OK, so the domain can't include -4 BUT why is it all real numbers greater than -4?

Answer: Domain is all real numbers greater than -4 - Nov 7th 2008, 09:47 AMjames_bond
Because in $\displaystyle \sqrt{x}$ $\displaystyle x$ must be greater than or equal to 0 (at least if you want it to be real..;))

- Nov 7th 2008, 09:47 AMmwok
- Nov 7th 2008, 09:51 AMjames_bond
Only if you're familiar with complex numbers, see here.

- Nov 7th 2008, 09:55 AMmwok
Ahh, thanks!

- Nov 7th 2008, 10:05 AMmwok
Hmmm, another question...how come this is different then?

f(x) = x / sqrt( abs(x) - 4 )

Answer: (-infinity, -4) U (4, infinity) - Nov 7th 2008, 10:47 PMjames_bond
$\displaystyle |a| = \begin{cases} a, & \mbox{if } a \ge 0 \\ -a, & \mbox{if } a < 0. \end{cases}$

Can you solve it now?