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## #1 2014-07-11 07:24:08

Al-Allo
Member
Registered: 2012-08-23
Posts: 324

### Limits at infinity

Hi, basic question here:

I have

lim 4z^2+z^6
_________
1-5z^3

x->positive infinity

Now, I know that this reduces to :

4/z+z^3
________=         0+infinity
_________
0-5
1/z^3 -5

which gives infinity over -5. Now, how do I evaluate infinity over a constant ??? Is there a fact or definition that tells us how to deal with this ?

Thank you

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## #2 2014-07-11 08:01:32

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 103,721

### Re: Limits at infinity

Hi;

The limit is of the form - ∞/∞ so you may use L'Hopitals rule on it.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
A number by itself is useful, but it is far more useful to know how accurate or certain that number is.

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## #3 2014-07-11 08:02:36

Al-Allo
Member
Registered: 2012-08-23
Posts: 324

### Re: Limits at infinity

I didn't learn that...  yet.

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## #4 2014-07-11 08:04:17

Al-Allo
Member
Registered: 2012-08-23
Posts: 324

### Re: Limits at infinity

Btw, if you want to see the problem, go her ; http://tutorial.math.lamar.edu/Classes/CalcI/LimitsAtInfinityI.aspx

it's example 4 at the end

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## #5 2014-07-11 08:04:18

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 103,721

### Re: Limits at infinity

Have you tried to break the rational function into smaller pieces?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
A number by itself is useful, but it is far more useful to know how accurate or certain that number is.

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## #6 2014-07-11 08:10:29

Maburo
Member
Registered: 2013-01-08
Posts: 287

### Re: Limits at infinity

L'Hopital's rule basically states that if

So you could differentiate the numerator and denominator until you no longer have 'z' in the denominator.

Last edited by Maburo (2014-07-11 08:11:46)

"Pure mathematics is, in its way, the poetry of logical ideas."
-Albert Einstein

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## #7 2014-07-11 08:26:54

Al-Allo
Member
Registered: 2012-08-23
Posts: 324

### Re: Limits at infinity

I didn't learn differentiation...

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## #8 2014-07-11 08:29:32

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 103,721

### Re: Limits at infinity

Divide the numerator and the denominator by z^3. Want to see how? You would then use:

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
A number by itself is useful, but it is far more useful to know how accurate or certain that number is.

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## #9 2014-07-11 08:40:46

Maburo
Member
Registered: 2013-01-08
Posts: 287

### Re: Limits at infinity

In the first post, that seems to have been done. Al-Allo reduced

"Pure mathematics is, in its way, the poetry of logical ideas."
-Albert Einstein

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## #10 2014-07-11 08:44:51

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 103,721

### Re: Limits at infinity

You are right, he is done then. - ∞ / 5 is -∞. That is the correct answer for the limit.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
A number by itself is useful, but it is far more useful to know how accurate or certain that number is.

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## #11 2014-07-11 08:46:39

Maburo
Member
Registered: 2013-01-08
Posts: 287

### Re: Limits at infinity

Yes, indeed.

"Pure mathematics is, in its way, the poetry of logical ideas."
-Albert Einstein

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## #12 2014-07-11 09:02:56

Al-Allo
Member
Registered: 2012-08-23
Posts: 324

### Re: Limits at infinity

But... how do you know that - ∞ / 5 is -∞ ???

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## #13 2014-07-11 09:05:04

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 103,721

### Re: Limits at infinity

http://www.vitutor.com/calculus/limits/ … inity.html

if the degree of z is higher on top, then the limit is infinity

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
A number by itself is useful, but it is far more useful to know how accurate or certain that number is.

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## #14 2014-07-11 09:13:39

Al-Allo
Member
Registered: 2012-08-23
Posts: 324

### Re: Limits at infinity

Ah ok, thanks for the web page.

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## #15 2014-07-11 09:15:50

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 103,721

### Re: Limits at infinity

Also, there is a theorem for when there is one polynomial over another. I put it into post #13.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
A number by itself is useful, but it is far more useful to know how accurate or certain that number is.

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## #16 2014-07-11 09:20:33

Al-Allo
Member
Registered: 2012-08-23
Posts: 324

### Re: Limits at infinity

bobbym wrote:

Also, there is a theorem for when there is one polynomial over another. I put it into post #13.

Yes, I saw it

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## #17 2014-07-11 11:15:30

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 103,721

### Re: Limits at infinity

Are you satisfied or do you need anything else?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
A number by itself is useful, but it is far more useful to know how accurate or certain that number is.

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## #18 2014-07-14 03:59:04

Al-Allo
Member
Registered: 2012-08-23
Posts: 324

### Re: Limits at infinity

Well, I have another question.

lim  9h
x->0

What do you do in these occasions ? Is there a definition that tells us the actions needed ? Thank you (I'm referring to the different letters.)

Last edited by Al-Allo (2014-07-14 03:59:26)

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## #19 2014-07-14 04:03:37

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 103,721

### Re: Limits at infinity

That would be 9h. Think about it, if x goes to 0 how does that affect 9h. It does not.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
A number by itself is useful, but it is far more useful to know how accurate or certain that number is.

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## #20 2014-07-14 04:08:44

Al-Allo
Member
Registered: 2012-08-23
Posts: 324

### Re: Limits at infinity

bobbym wrote:

That would be 9h. Think about it, if x goes to 0 how does that affect 9h. It does not.

Well, at first i thought the same thing but wasn't too sure... ok thanks

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## #21 2014-07-14 04:09:45

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 103,721

### Re: Limits at infinity

You could consider that 9h to be a constant and the limit operator does not affect it.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
A number by itself is useful, but it is far more useful to know how accurate or certain that number is.

Offline

## #22 2014-07-14 04:19:12

Al-Allo
Member
Registered: 2012-08-23
Posts: 324

### Re: Limits at infinity

Yeah, that would be an idea!

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