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Homework Help: Integral problem

  1. Mar 21, 2007 #1

    dnt

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    1. The problem statement, all variables and given/known data

    how do you integrate (xcox)dx

    2. Relevant equations

    n/a

    3. The attempt at a solution

    ive gone through so many ways that ive learned how to integrate problems and i cannot figure this one out. can someone just give me a hint on how to start it? thanks. (i have a feeling this is easy and im overlooking a really basic way of doing it)
     
  2. jcsd
  3. Mar 21, 2007 #2

    VietDao29

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    Homework Helper

    When seeing a product of a polynomial and ex, or some kind of trigonometry functions. One should think about Integrate by Parts.
    By letting u = the polynomial, in this case u = x.
    And dv = the rest.
    By the way, is your problem:
    [tex]\int x \cos x dx \quad \mbox{or} \quad \int x \cot x dx[/tex]?
    Can you go from here? :)
     
  4. Mar 21, 2007 #3
    Integration by parts.
     
  5. Mar 21, 2007 #4
    I do not know if this mnemonic is taught everywhere, but when I was in school learning integration by parts, we were asked to remember ILATE, without justification, when deciding which part is to be u and which is to be dv. Of course, this rule need not work every time.

    Inverse(Trigonometric)-Logarithmic-Algebraic-Trigonometric-Exponential.

    The one that comes before the other will be u.
     
  6. Mar 21, 2007 #5

    dnt

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    thanks. i knew it was easier than i thought.

    and btw it was cos (forgot the s)
     
  7. Mar 21, 2007 #6

    VietDao29

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    Homework Helper

    Well, yes, some of the textbooks here do mention it. However, the are very rare, I think.
    Btw, I don't know what it's called in English. Since, I am not a native-English speaker. Still have to learn a lot. :blushing:
    So yeah, thanks for the info. :)
     
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