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Prove that x+y divides x^n + y^n for all n which is odd

*Last edited by Agnishom (2014-05-09 01:24:29)*

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

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**bob bundy****Moderator**- Registered: 2010-06-20
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I can show a proof by induction. It'll take a while to Latex it.

Bob

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You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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Induction, hmm.

Thanks for the hint. I'll try it myself.

'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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**bob bundy****Moderator**- Registered: 2010-06-20
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QED.

Bob

*Last edited by bob bundy (2014-05-09 05:55:42)*

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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Thanks. Can you prove this:

If a,b are positive integers:

And if a + b + ab = 2020

then a+b=88?

'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
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**In mathematics, you don't understand things. You just get used to them.**

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

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**bob bundy****Moderator**- Registered: 2010-06-20
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Maybe like this:

Find the (only two) factors of 2021. (other than 1 and 2021)

Bob

*Last edited by bob bundy (2014-05-09 06:27:17)*

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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Thanks, bob

What would Ramanujan do bobbym? What is the D command?

By the way, FindInstance has already given me 42 and 46

'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
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One of Ramanujan's great answers was when he reduced a problem to 7 possibilities and tried them all! In the age before computers, that guy was doing experimental math.

That is the divisors operator. Notice that they are paired up to product to 2020 - 88. Now just find 2 that sum to 88. They are easily found.

**In mathematics, you don't understand things. You just get used to them.**

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

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I've got FindInstance. Why must I work so hard?

'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
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Because I want to show the work. Those divisors can be done by hand.

I would take Bob's idea but you are still going to have to factor a number.

**In mathematics, you don't understand things. You just get used to them.**

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

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What work do you want to show?

I'll factor it with a computer

'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
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I hid that answer because it is a last resort and much better than having no answer.

There is an amusing story of the mathematician who walked up to the board wrote this:

then he did the multiplication. Took 10 minutes to do, never said a word and just sat down. That was his proof that 2^64+1 is composite.

I thjought post #7 was good for an answer, so take that one.

**In mathematics, you don't understand things. You just get used to them.**

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

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I cannot do that. I will most certainly make mistakes.

'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
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Here is another way that uses only algebra:

Substituting for b using the 2nd equation.

That is a quadratic that is easily solved and gets a =42 and a = 46.

So we have a = 42 and b = 46 or a = 46 and b = 42.

**In mathematics, you don't understand things. You just get used to them.**

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

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Consider

This is a G.P. with common ratio

so we haveBut *n* is odd so

Multiplying both sides by

givesSince the LHS is an integer, the RHS must be an integer. QED.

*Last edited by Nehushtan (2014-05-09 08:16:02)*

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**bob bundy****Moderator**- Registered: 2010-06-20
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bobbym wrote:

I would take Bob's idea but you are still going to have to factor a number.

Earlier I number crunched it in Excel, but further thought provides immediate factors:

Factors of 2021 (assume just two to provide a unique solution)

To end in 1 the number pair must come from one of these sets

{11,21,31,41} {19,29,39} {13,23,33,43 with 17,27,37,47} [anything over 50 is too big]*

You can dismiss all of the first as 41 x 41 is only just over 1600.

You can dismiss the second as 39 x 39 < 1600.

So any factor pair must come from pairing a number ending in 3 with a number ending in 7

37 x 43 < 40 x 45 = 1800 so only 47 need be considered.

47 x 33 < 50 x 35 = 1750 so you only need to try 43 x 47 by long multiplication.

Bob

* Just realised something like 37 x 53 might work .... working on this modification

*Last edited by bob bundy (2014-05-09 08:45:17)*

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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**bob bundy****Moderator**- Registered: 2010-06-20
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OK. Above is wrong although you could still get that answer quickly without lots of number crunching.

However for a complete analysis see the screen shot below. I think it is self explanatory.

Bob

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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**zetafunc.****Guest**

Agnishom wrote:

Thanks. Can you prove this:

If a,b are positive integers:

And if a + b + ab = 2020

then a+b=88?

**zetafunc.****Guest**

zetafunc. wrote:

Agnishom wrote:Thanks. Can you prove this:

If a,b are positive integers:

And if a + b + ab = 2020

then a+b=88?

You may have noticed that I ignored the cases a = 0, b = 2020 and b = 0, a = 2020. Your constraint was that a and b are *positive*, hence we can ignore these cases.

**zetafunc.****Guest**

There are also the cases for negative a and b, but, again, these do not satisfy the constraints of your problem.

bobbym wrote:

Here is another way that uses only algebra:

Substituting for b using the 2nd equation.

That is a quadratic that is easily solved and gets a =42 and a = 46.

So we have a = 42 and b = 46 or a = 46 and b = 42.

You are asked to prove that a+b = 88, You cannot begin by accepting that fact

'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
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You can accept the statement and work backwards like a Reductio ad Absurdum. Then you try to work towards a contradiction which in this case does not happen. But I will ask for some other opinion 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.**

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In reductio ad absurdum, you do not accept the statement. You and bob had a huge argument about this process somewhere but I do not remember

'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
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In reductio ad absurdum, you do not accept the statement.

I think you do.

anonimnystefy vindicated the idea of working forward back then. I will ask the mathematicians what they think.

**In mathematics, you don't understand things. You just get used to them.**

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

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