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**Mathisweird****Member**- Registered: 2014-02-20
- Posts: 1

My issue is why 10x why not (infinite)x. The rule is multiply 10 by each number after the decimal point but if .9999 repeating then you multiply by infinite?

Infinite x - x = infinite -x

Infinite x = infinite

X= infinite/infinite = undefined

So if Infinite/infinite=1 then 0.99999=1

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 96,606

Hi;

why not (infinite)x.

What exactly does that mean? You can not say ∞ * x because infinity is not a number.

The rule is multiply 10 by each number after the decimal point

What rule?

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That would be a bad idea. ∞ is not a normal number.

Multiplying and dividing with ∞ will cripple all your equations.

For example,

x + 2 = 0

⇒ x = -2

If we tried t multiply and divide by ∞

x + 2 = 0

⇒ ∞(x + 2) = ∞ 0

⇒ ∞x + ∞ = Indeterminate

⇒ x = (Indeterminate-∞)/∞ = Indeterminate

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Rule Dividing by ∞ is not allowed.

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'

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**niharika_kumar****Member**- From: Numeraland
- Registered: 2013-02-12
- Posts: 1,056

Agnishom wrote:

Rule Dividing by ∞ is not allowed.

from where did this rule come? we divide 1 by infinity in physics for solving some of the numericals in lens, as it is the far point of the eye.

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**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 15,937

You NEVER divide by infinity. More precisely, you NEVER use infinity as a number. What you do in physics class is look at the limit of the expression as one of the distances tends to infinity. You just use it intuitively without notation.

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 7,056

Mathisweird wrote:

So if Infinite/infinite=1 then 0.99999=1

All that about not treating infinity as a number plus this.

Why not:

So if Infinite/infinite=67.234 then 0.99999=67.234

and then 67.234 = 1 => 66.234 = 0 etc etc

Whoops. That's why you cannot treat infinity as a number.

Bob

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niharika_kumar wrote:

Agnishom wrote:Rule Dividing by ∞ is not allowed.

from where did this rule come? we divide 1 by infinity in physics for solving some of the numericals in lens, as it is the far point of the eye.

In the physics class, you also divide by 0 to get infinity but that is also finding the limit like stefy said

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'

'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

'But our love is like the wind. I can't see it but I can feel it.' -A Walk to remember

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 96,606

Some CAS also did that. I remember Mupad, can you guess who was instrumental in putting it into that CAS?

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**ShivamS****Member**- Registered: 2011-02-07
- Posts: 3,646

From what I have read, operations on infinity depends on the person. For example, according to Rudin, all primitive operations on infinity are defined.

*Last edited by ShivamS (2014-02-21 14:00:09)*

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bobbym wrote:

Some CAS also did that. I remember Mupad, can you guess who was instrumental in putting it into that CAS?

Who? You? JaneFairFax? Anthony R Babbage?

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 96,606

It was me of course.

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**If it ain't broke, fix it until it is.**

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It is a bad idea

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 96,606

From the engineering point of view it saves time. But mathematically it is incorrect.

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**If it ain't broke, fix it until it is.**

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**niharika_kumar****Member**- From: Numeraland
- Registered: 2013-02-12
- Posts: 1,056

oh, thanks.

friendship is tan 90°.

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'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

'But our love is like the wind. I can't see it but I can feel it.' -A Walk to remember

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He is not GiB?

'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

'But our love is like the wind. I can't see it but I can feel it.' -A Walk to remember

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 96,606

This one is left open.

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**If it ain't broke, fix it until it is.**

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