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Homework Help: Perpetual integration

  1. Mar 18, 2010 #1
    1. somebody please help me:
    I have to integrate the following:

    https://www.physicsforums.com/attachment.php?attachmentid=24485&d=1268939055

    as you can see I have an answer - this is such a long problem - it's like an integration of an integration of an...
    and the final answer gives a math error in the calculator.

    I also get a value out of calculator of -7.169337845

    thank for any advice
     
  2. jcsd
  3. Mar 18, 2010 #2
    I am unsure of what the problem is. What are you trying to integrate? I see "I have to integrate the following:" and nothing following.
     
  4. Mar 18, 2010 #3
    you cant see the problem on the image?
     
  5. Mar 18, 2010 #4

    gabbagabbahey

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    No. I used the quote button on your original post and it looks like you are trying to post an image just by putting image tags around an http address for an attachment. I don't think that works even if the http address links to an image in jpeg form. To make things worse, it looks like the attachment http address is invalid.

    Try uploading your image to a free image hosting site like imageshack.us and then just posting the forum hotlink url that they give you.

    Either that or take the time to type out your problem statement and the key lines in your attempt like everybody else does.
     
  6. Mar 18, 2010 #5
    I'll ask you to leave your sarcasm for another arena please.
    This was typed out properly in mathType.
     
  7. Mar 18, 2010 #6

    gabbagabbahey

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    I wasn't being sarcastic. This forum supports [itex]\LaTeX[/itex] and it is often quicker for you to type it out here than to type it up in a separate document and then post it as an attachment or upload it to an image hosting site.
     
  8. Mar 18, 2010 #7
    sorry - it's been a long week. I will look into it:bugeye:
     
  9. Mar 18, 2010 #8
    [itex]

    \[
    \begin{array}{l}
    \int_0^3 {2x^3 e^{ - x} dx} \\
    = \frac{{13\left( { - 12 - e^3 } \right)}}{{e^3 }} \\
    \end{array}
    \]

    [/itex]
     
  10. Mar 18, 2010 #9
    tricky
     
  11. Mar 18, 2010 #10

    gabbagabbahey

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    there are two different types of tags you can use...the ones I used were "itex" which is short for inline LaTeX and is only good for short expression that you want to appear inline with the rest of your text. For larger expressions, just use "tex" tags.

    [tex]\begin{aligned}\int_0^3 2x^3 e^{ - x} dx & = \left.-6x^2e^{-x}\right|_0^3 +\int_0^3 6x^2 e^{ - x} dx\\ & = \left.-6x^2e^{-x}\right|_0^3+ \left.12xe^{-x}\right|_0^3 + \int_0^3 12x e^{ - x} dx \\ & = \frac{{13\left( { - 12 - e^3 } \right)}}{e^3}\end{aligned}[/tex]

    Were you trying to post something like this? You can click on the image to see the code that generated it (or quote my post to see it without all kinds of extra spaces and line-breaks).
     
  12. Mar 18, 2010 #11
    thank you gabbagabbahey
    I will take a look at it over the weekend.

    Is that the correct finished solution you posted above?
     
  13. Mar 18, 2010 #12

    gabbagabbahey

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    No.

    Is that the problem you are trying to calculate?
     
  14. Mar 18, 2010 #13
    yes
    it is the problem
    but it went on and on and...
     
  15. Mar 18, 2010 #14

    gabbagabbahey

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    It shouldn't go too long. Just use integration by parts (IBP) 3 times.

    Why not show me your first iteration of IBP...
     
  16. Mar 18, 2010 #15
    i have attached a .pdf file of my written workings
    hope you can read my scribbles
     

    Attached Files:

  17. Mar 18, 2010 #16
    it is all part of this attached .pdf file.
    I have succeeded to complete part one of the question.
     

    Attached Files:

  18. Mar 18, 2010 #17

    Mark44

    Staff: Mentor

    Changed your [ itex] tags, and got rid or your array stuff.
     
  19. Mar 18, 2010 #18

    gabbagabbahey

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    It looks like you've done all the individual iterations of IPB properly (although you've neglected to write the integration limits in at each step).

    You're problem is with the arithmetic after you substitute in your limits of integration:

    [tex]\begin{aligned}\int_0^3 2x^3 e^{ - x} dx & = \left.-2x^3e^{-x}\right|_0^3- \left.6x^2e^{-x}\right|_0^3 - \left.12xe^{-x}\right|_0^3-\left.12e^{-x}\right|_0^3 \\ & = \left.-2e^{-x}\left(x^3+3x^2+6x+6\right)\right|_0^3 \\ & = -2e^{-3}\left(27+27+18+6\right)+2e^{0}\left(0+0+0+6\right) \\ & \neq \frac{{13\left( { - 12 - e^3 } \right)}}{e^3}\end{aligned}[/tex]
     
  20. Mar 18, 2010 #19

    Mark44

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    IPB - integration py barts?:smile:
     
  21. Mar 18, 2010 #20

    gabbagabbahey

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    My calculus teacher's name was Ms. Edna Krabappel o:)
     
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