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Homework Help: Complex exponential help please

  1. Oct 3, 2005 #1
    Express the following in the form [tex]z=Re[Ae^{i(\omega t+\alpha)}][/tex]

    [tex]z=cos(\omega t - \frac{\pi}{3}) - cos (\omega t)[/tex]
    and
    [tex]z=sin(\omega t) - 2cos(\omega t - \frac{\pi}{4}) + cos(\omega t)[/tex]

    I got a few of the problems correct by using trig. identities but it was pretty tough and two I can't get. Our teacher said you can use a tric to solve them easier but didn't have time to finish, I just know it has something to do with the polar form of [tex]e^{i \theta}[/tex] I really have no clue on how to do these without using the really long method of trig. identities. Any help would be great. Thanks
     
    Last edited: Oct 3, 2005
  2. jcsd
  3. Oct 4, 2005 #2

    CarlB

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    [tex] z = \textrm{Re}(e^{i(\omega t - \pi/3} - e^{i\omega t})[/tex]
    [tex] =\textrm{Re}( e^{i\omega t}( e^{-i\pi/3}-1))[/tex]
    [tex] = \textrm{Re}(( e^{-i\pi/3}-1) \;\;e^{i\omega t})[/tex]

    Does that help?

    Don't be ashamed, it's far better to be conversant in trig than to know a few tricks.

    Carl
     
  4. Oct 4, 2005 #3

    robphy

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    Along the lines of CarlB, but without jumping straight in to using Re(),
    recall [tex]e^{i\theta}=\cos\theta+i\sin\theta[/tex], from which you can derive
    [tex]\cos\theta=\frac{1}{2}(e^{i\theta}+e^{-i\theta})[/tex] and a similar expression for [tex]\sin\theta[/tex] (which I left for you to do).

    So, now:
    [tex]
    z&=\frac{1}{2}(e^{i[\omega t-\pi/3]}+e^{-i[\omega t-\pi/3]})-\frac{1}{2}(e^{i[\omega t]}+e^{-i[\omega t]})[/tex]
    then do some algebra.

    Recall that [tex]Re(z)=\frac{1}{2}(z+z^*)[/tex]. Thus [tex]Re(e^{i\theta})=\cos\theta[/tex].
     
  5. Oct 4, 2005 #4
    Thanks a lot, that makes it much more simple.
     
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