# [SOLVED] Domain

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• Sep 23rd 2008, 07:39 PM
>_<SHY_GUY>_<
[SOLVED] Domain
I Am Getting Confused on How To Find The Domain And Range Of Functions of a Graph.

For Example: f(x)=x^4-8x^2+8.

it has two min. points and one max point.
• Sep 23rd 2008, 07:43 PM
icemanfan
A function that is a single polynomial will have a domain of all real numbers. For the polynomial function you specified, you must compute the absolute minimum of the function in order to determine the range. The absolute minimum will be the smaller of the two local minima.
• Sep 23rd 2008, 07:45 PM
Jhevon
Quote:

Originally Posted by >_<SHY_GUY>_<
I Am Getting Confused on How To Find The Domain And Range Of Functions of a Graph.

For Example: f(x)=x^4-8x^2+8.

it has two min. points and one max point.

for a function of x:

the domain is the set of x-values for with the function is defined.

the range is the set of y-values for which the function is defined.

it is often easier to find the values for which a function is not defined, and say the domain or range is everything but that

with polynomials though, our lives are easy. polynomials are defined for all real x (unless we are restricting the domain with some additional condition).

for an even polynomial where the leading coefficient is positive, the range is $y \in [\text{absolute minimum}, \infty)$
• Sep 23rd 2008, 07:47 PM
>_<SHY_GUY>_<
Quote:

Originally Posted by icemanfan
A function that is a single polynomial will have a domain of all real numbers. For the polynomial function you specified, you must compute the absolute minimum of the function in order to determine the range. The absolute minimum will be the smaller of the two local minima.

The Abs. Min. Is at 2,-8 and -2,-8... But There Is A Max. Does That Affect the Range?

So Would The Range be [-8, Infinity) ?
• Sep 23rd 2008, 07:52 PM
Len
2a. Domain and Range of a Function
Quote:

Domain: The complete set of possible values of the independent variable in a function is called the domain of the function.
In plain English, the definition means:
The domain of a function is all the possible x values which will make the function "work" by outputting real values.
When finding the domain, remember:

• denominator (bottom) of a fraction cannot be zero
• the values under a square root must be positive
Range: The complete set of all possible resulting values of the dependent variable in a function is the range of the function.
In plain English, the definition means:
The range of a function is the possible y values of a function resulting when we substitute all the possible x-values.
When finding the range, remember:

• substitute different x-values into the expression for y to see what is happening
• make sure you look for minimum and maximum values of y
• draw a sketch!

In your case the Domain would be all Real numbers.
• Sep 23rd 2008, 07:54 PM
Jhevon
Quote:

Originally Posted by >_<SHY_GUY>_<
The Abs. Min. Is at 2,-8 and -2,-8... But There Is A Max. Does That Affect the Range?

So Would The Range be [-8, Infinity) ?

yes

the max does not affect the range. it is a local max and the function increases beyond it at the ends
• Sep 23rd 2008, 07:55 PM
>_<SHY_GUY>_<
Quote:

Originally Posted by Jhevon
for a function of x:

the domain is the set of x-values for with the function is defined.

the range is the set of y-values for which the function is defined.

it is often easier to find the values for which a function is not defined, and say the domain or range is everything but that

with polynomials though, our lives are easy. polynomials are defined for all real x (unless we are restricting the domain with some additional condition).

for an even polynomial where the leading coefficient is positive, the range is $y \in [\text{absolute minimum}, \infty)$

Domain Of The Graph Would Be -infinity, infinity.... but i just dont see it...

same with x^3. the domain and range is all real numbers... i just get confused
• Sep 23rd 2008, 07:56 PM
Jhevon
Quote:

Originally Posted by Len
2a. Domain and Range of a Function

In your case the Domain would be all Real numbers.

a slight tweek here.

the values under a square root must be non-negative, since we can include zero, which is not a positive (or negative) number
• Sep 23rd 2008, 07:58 PM
Jhevon
Quote:

Originally Posted by >_<SHY_GUY>_<
Domain Of The Graph Would Be -infinity, infinity.... but i just dont see it...

same with x^3. the domain and range is all real numbers... i just get confused

how are you confused?

no matter what x-values you plug in, the function works. you cover all x-values and so the domain is all real numbers. as for x^3, the function is continuous. one end goes to infinity, the other goes to minus infinity. you cover all y-values. so the domain is all real numbers.
• Sep 23rd 2008, 07:58 PM
>_<SHY_GUY>_<
in other words. if the graph shows the x-values growing continuously...then the range of a graph is all real
• Sep 23rd 2008, 08:04 PM
icemanfan
Range can be tricky. Remember that for polynomials with real coefficients and odd degree, the range is all real numbers. For polynomials of even degree, you have to find the absolute minimum if the coefficient of the highest power of x is positive, or the absolute maximum if the coefficient of the highest power of x is negative.
• Sep 23rd 2008, 08:05 PM
Len
Quote:

Originally Posted by >_<SHY_GUY>_<
to see the range, i know i must draw a graph, usually. but how is it that you can see it? domain i get now. range is just puzzling me now

The range is all possible values of y. Since you have said that -8 is the absolute minimum, there can be no values of y under -8 for any value of x.

Take the limit as x goes to positive and negative infinity. They both go to positive infinity. So like you said the range would be -8 to positive infinity.
• Sep 23rd 2008, 08:08 PM
>_<SHY_GUY>_<
Quote:

Originally Posted by icemanfan
Range can be tricky. Remember that for polynomials with real coefficients and odd degree, the range is all real numbers. For polynomials of even degree, you have to find the absolute minimum if the coefficient of the highest power of x is positive, or the absolute maximum if the coefficient of the highest power of x is negative.

OH!(Surprised)

So Odd=All Real.
Even: You Have to find out the Abs. Min/Max, And The Y-value For That Point.

Example: X^2. [0, Infinity)
if the Vertex Moves From The Point, the range moves as well?

Correct?
• Sep 23rd 2008, 08:12 PM
icemanfan
Quote:

Originally Posted by >_<SHY_GUY>_<
OH!(Surprised)

So Odd=All Real.
Even: You Have to find out the Abs. Min/Max, And The Y-value For That Point.

Example: X^2. [0, Infinity)
if the Vertex Moves From The Point, the range moves as well?

Correct?

Correct.
If you change $x^2$ to $x^2 + 1$, the range will change accordingly. However, note that the ranges of $x^2$ and $(x-1)^2$ are the same, despite having different vertices.
• Sep 23rd 2008, 08:17 PM
>_<SHY_GUY>_<
Quote:

Originally Posted by icemanfan
Correct.
If you change $x^2$ to $x^2 + 1$, the range will change accordingly. However, note that the ranges of $x^2$ and $(x-1)^2$ are the same, despite having different vertices.

Thank You(Happy)

That will help me for my test tomorrow :D
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