# Whats the domain?????

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• August 20th 2008, 05:54 PM
sadieann
Whats the domain?????
• August 20th 2008, 06:16 PM
arbolis
The domain is all the $x$ that make $x(x-18)>0$.
• August 20th 2008, 06:19 PM
sadieann
I know that..but i need the domain in interval notation of that function
• August 20th 2008, 06:25 PM
Chris L T521
Quote:

Originally Posted by arbolis
The domain is all the $x$ that make $x(x-18)>0$.

Quote:

Originally Posted by sadieann
I know that..but i need the domain in interval notation of that function

Well, if you know this is the case, then for what values of x is this inequality true? Just solve the inequality. Its fairly straightforward. Once you solve the inequality, the intervals become pretty obvious.

--Chris
• August 20th 2008, 06:25 PM
arbolis
Ok. Note that $x(x-18)$ vanishes when x equals 0 or 18. Note also that $y=x(x-18)=x^2-18x$ is an equation of a parabola with a global minimum. That mean that $x(x-18)\geq 0$ when $x \in (-\infty,0]$ U $[18,+\infty,)$, that's your domain.
EDIT: I'm sorry in my first post, it should be "The domain is all the $x$ that make $x(x-18)\geq 0$."
• August 20th 2008, 06:38 PM
sadieann
Thank you so much man.
• August 20th 2008, 06:43 PM
arbolis
A pleasure to help. I understand you were in a rush but when you have time, try to understand what I did and ask us if you don't understand. Good luck.