# Domain and Range Functions

• Jun 4th 2009, 08:30 PM
sgonzalez90
Domain and Range Functions
I am to write this in interval notation. I don't remember learning this:
But here are two examples. On a graph, i know how to get the domain and range, but not from a function.

• Jun 4th 2009, 08:48 PM
VonNemo19
Quote:

Originally Posted by sgonzalez90
I am to write this in interval notation. I don't remember learning this:
But here are two examples. On a graph, i know how to get the domain and range, but not from a function.

Recall that the domain of a function is all possibilities of x at which f(x) is defined:

$f(x)=-4-\sqrt{9-x^2}$
where is this function defined? Well, you know that you can't take the sqrt of a negative number, s f(x) is defined every where that $9-x^2\geq0\Rightarrow\mid{x}\mid\leq{3}$. (If x where any bigger than 3, 9-x^2 would be less than zero. A big no no unless we're considering complex numbers.)

In interval notation: $[-3,3]$

you can do the other now can't you?
• Jun 4th 2009, 09:02 PM
sgonzalez90
Quote:

Originally Posted by VonNemo19
Recall that the domain of a function is all possibilities of x at which f(x) is defined:

$f(x)=-4-\sqrt{9-x^2}$
where is this function defined? Well, you know that you can't take the sqrt of a negative number, s f(x) is defined every where that $9-x^2\geq0\Rightarrow\mid{x}\mid\leq{3}$. (If x where any bigger than 3, 9-x^2 would be less than zero. A big no no unless we're considering complex numbers.)

In interval notation: $[-3,3]$

you can do the other now can't you?

great, I understand the concept. But how do we solve for the range?
• Jun 4th 2009, 10:09 PM
VonNemo19
Solve for x and then determine what y cannot be:

$y=-4-\sqrt{9-x^2}$

$y+4=-\sqrt{9-x^2}$

$(-y-4)^2=9-x^2$

$(-y-4)^2-9=-x^2$

$\pm\sqrt{-y^2-8y-7}=x$

$\pm\sqrt{-[(y+7)(y+1)]}=x$

This one requires alittle more thought..........

When is the expression under the radical sign negative?
• Jun 4th 2009, 10:43 PM
yeongil
For range I usually look at its graph, or plug values of x into my head.

$y=-4-\sqrt{9-x^2}$

The domain is [-3, 3]. If you look at $\sqrt{9-x^2}$ only, what is its minimum value? It's 0, when x is -3 or 3. The maximum value? It's 3, when x = 0.

Now consider the whole function. If you plug in x = -3 or 3, you actually get the maximum value of the function, because there is a negative in front of the square root:
$y=-4-\sqrt{9-9} = -4$
Plug in 0 for x to get the minimum value of the function:
$y=-4-\sqrt{9-0} = -7$
The range is [-7, -4].

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