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**White_Owl****Member**- Registered: 2010-03-03
- Posts: 99

In my homework for this week I have a question:

If I do a substitution *u=3x+25*, then I have

Which does not help much.

If I try to find such *u* that du will eliminate x from numerator, then: *du=1/x dx* and *u *have to be *ln(x)*

Which is also not very helpful...

Finally I run the original integral through Maxima, and it gave me:

Which is very scary and I have no idea how it come to this formula.

So which "u" should I use? How to find such "u"? Or maybe there is another approach I should use? I am completely stuck.

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 14,845

Hi White_Owl

Continuing on the first thing you tried:

Can you continue from here?

*Last edited by anonimnystefy (2013-01-24 08:49:57)*

Here lies the reader who will never open this book. He is forever dead.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

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**White_Owl****Member**- Registered: 2010-03-03
- Posts: 99

Ok... Then:

And after substituting back

But now if I differentiate it back:

Where did 9 come from?

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 14,845

Hi White_Owl

You have multiplied by 3 instead of dividing by 3 when making the substitution...

Here lies the reader who will never open this book. He is forever dead.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

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**White_Owl****Member**- Registered: 2010-03-03
- Posts: 99

Yes, it does look promising.

Thank you very much.

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 14,845

You're welcome!

Here lies the reader who will never open this book. He is forever dead.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

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