# Whats the domain?????

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

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

Quote:

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
• Aug 20th 2008, 06:25 PM
arbolis
Ok. Note that $\displaystyle x(x-18)$ vanishes when x equals 0 or 18. Note also that $\displaystyle y=x(x-18)=x^2-18x$ is an equation of a parabola with a global minimum. That mean that $\displaystyle x(x-18)\geq 0$ when $\displaystyle x \in (-\infty,0]$ U $\displaystyle [18,+\infty,)$, that's your domain.
EDIT: I'm sorry in my first post, it should be "The domain is all the $\displaystyle x$ that make $\displaystyle x(x-18)\geq 0$."
• Aug 20th 2008, 06:38 PM