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Complex number in integral

  1. Aug 14, 2007 #1
    Suppose we want to find

    [tex] \int e^x \cos{x} \ dx [/tex]

    We know from [tex] e^{ix} = \cos{x} + i\sin{x} [/tex] that the real part of [tex] e^{ix} [/tex] equals [tex] \cos{x} [/tex]. So suppose we want to find that integral, is it ok to study the real part of [tex] e^x \cdot e^{ix} [/tex]? In that case we get

    [tex] \int e^x \cos{x} \ dx = \int e^x e^{ix} \ dx = \frac{e^x e^{ix}}{1+i}[/tex]

    Doing this gives us
    [tex] (1/2) e^x e^{ix} (1 - i)[/tex]
    [tex] (1/2) e^x (\cos{x} + i\sin{x})(1 - i)[/tex]
    [tex] (1/2) e^x (\cos{x} - i\cos{x} + i\sin{x} + \sin{x})[/tex]

    Hence we find that

    [tex] \int e^x \cos{x} \ dx = (1/2)e^x (\cos{x} + \sin{x}) [/tex]

    Which is indeed the result we get from using integration by parts. We also get that the imaginary part is (same as by integration by parts)

    [tex] \int e^x \sin{x} \ dx = (1/2) e^x (\sin{x} - \cos{x}) [/tex].

    But is this technique ok and is there any more examples of this technique? Is there a name for this? Doing this instead of using integration by parts we get the integrals of [tex] e^x \cos{x} [/tex] and [tex] e^x \sin{x} [/tex] at the same time ... I can't think of any way why this shouldn't be ok.
     
  2. jcsd
  3. Aug 14, 2007 #2

    morphism

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    That's a completely valid technique.
     
  4. Aug 14, 2007 #3
    just one thing about notation.
    it's not correct to equate that the integral of e^x(1+i) eqauls the integral of e^xcos(x), but rather that the real part of the former intergal differs from the latter by a cosntant.
    besides this, looks like a nice idea, which can be very useful.
     
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