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**Fruityloop****Member**- Registered: 2009-05-18
- Posts: 120

This integral comes from trying to get the surface area of y=1/x rotated about the x-axis.

evaluate the integral

Good luck! It's a toughie.

The eclipses from Algol (an eclipsing binary star) come further apart in time when the Earth is moving away from Algol and closer together in time when the Earth is moving towards Algol, thereby proving that the speed of light is variable and that Einstein's Special Theory of Relativity is wrong.

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**noelevans****Member**- Registered: 2012-07-20
- Posts: 236

Problems of this nature (integrals obtained by rotation as mentioned) if they are amenable to integral tables or substitution methods are usually highly rigged. Rotation problems like this with just any old function f(x) usually generate horrendous integrands not amenable to elementary methods.

This may be one of those "bad" ones. I tried several different substitutions (algebraic, trigonometric, integration by parts) and got nowhere.

Good luck with it!

Writing "pretty" math (two dimensional) is easier to read and grasp than LaTex (one dimensional).

LaTex is like painting on many strips of paper and then stacking them to see what picture they make.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 88,714

Hi Fruityloop;

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

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,603

Hi bobbym

Check your first step after the v-substitution.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 88,714

Hi;

I am not sure where you mean.

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

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,603

Sorry, mybad. I forgot the substitution of differentials.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 88,714

That is okay.

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

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,603

What does M spit out for the integral?

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 88,714

The wrong answer!

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

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,603

Can you post it? I am interested to see what it is because you suggested using it.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 88,714

I hesitate to put his ugly solution that is magic next to mine.

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

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,603

Hi bobbym

Wouldn't the first term simplify to 1/2 *arcsinh(x**2) ?

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 88,714

Hi;

M's answer? I do not know. I only know that it comes out of nowhere. When I follow his steps I do not get that. Also the answer I get is wrong. That is why I did it my way. It is longer but at least I think it is correct.

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

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,603

Do the integral in Wolfram and click show steps. Interesting behaviour.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 88,714

Yes, that is what he gets. When I follow along the answer is different. Also it has a complex fungus attached to it!

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

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,603

Yes, the answer is different from what he gets in the "Show steps" solution! But I do not see any complex stuff.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 88,714

Did you work towards the solution using M? You will get the wrong answer.

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

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,603

Wolfram spits out the wrong answer.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 88,714

Is that a question? The package and Alpha are not the same thing. He gets the wrong answer on the integral because of the partial fraction decomposition. When I plug in x =1. he spit out a complex number!

Maple on the other hand in this case does the integration correctly.

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

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,603

But I don't have M.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 88,714

Well, then why did you want to see what it did?

I only mentioned the packages here for completeness. The whole thing can be done by hand.

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

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**noelevans****Member**- Registered: 2012-07-20
- Posts: 236

hi bobbym,

In going from the second step of your answer in post #3 to the third step did you miscopy the x^5 as an x^4? Should dx = ((-1/4)x^5)du?

Writing "pretty" math (two dimensional) is easier to read and grasp than LaTex (one dimensional).

LaTex is like painting on many strips of paper and then stacking them to see what picture they make.

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,603

Probably is.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 88,714

Hi noelevans;

Yes it is. I am fixing the above. Thanks.

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

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,603

Minor error compared to M's. Speaking of it, are you going to feedback them to llet them know about it?

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