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#1 2010-07-15 19:46:37

viorelbadicu
Member
Registered: 2010-07-15
Posts: 1

Very difficult Integral... Please Help

Could somebody help me with this integral??? I need it as soon as possible

∫(exp(-ax)*cos(b*x))dx


Thanks a lot

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#2 2010-07-15 19:51:42

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

Re: Very difficult Integral... Please Help

Hi viorelbadicu;

Hi;

From Pierce's short table of integrals # 431.



In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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#3 2010-12-27 08:49:11

soroban
Member
Registered: 2007-03-09
Posts: 452

Re: Very difficult Integral... Please Help

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#4 2011-01-04 20:48:16

Hello soroban
Guest

Re: Very difficult Integral... Please Help

I tried to solve it by parts but instead of using cos(bx) as u, I chose e^(-3x) and cos(bx) as my dv. I got a completely different answer but I would like to know if what I got is correct. My messy answer was: e^(-ax)[ 1/(b^2+a^2)sin(bx)]-[a/(b^3++ba^2]cos(bx)+C. Sorry for all the parenthesis. Thank you:(

#5 2011-01-04 20:58:53

Cosmopolite
Member
Registered: 2011-01-04
Posts: 6

Re: Very difficult Integral... Please Help

It was me that made the precedent post. I redid the work correctly using e^(-3x) as my u and cos(bx) as my dv and I got the same answer than you guys.

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#6 2011-01-04 21:00:02

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

Re: Very difficult Integral... Please Help

Hi Cosmopolite;

That is very good and welcome to the forum!


In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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#7 2011-01-05 13:05:34

Cosmopolite
Member
Registered: 2011-01-04
Posts: 6

Re: Very difficult Integral... Please Help

Thank you.

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