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Euler's identity question

  1. May 29, 2013 #1
    Hello. Please tell me how do I derive:

    cos(θ)=1/2 ([itex]e^{iθ}[/itex]+[itex]e^{-iθ}[/itex])


    [itex]e^{iθ}[/itex]=cos(θ) + isin(θ)

    as well as:

    sin(θ)=1/2i ([itex]e^{iθ}[/itex]-[itex]e^{-iθ}[/itex])

    I can't figure it out...for example, where does the 1/2 come from? Thank you:smile:
  2. jcsd
  3. May 29, 2013 #2
  4. May 29, 2013 #3


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    Science Advisor

    Write out what ##e^{-i\theta}## is.

    Edit: ninja'd by the OP
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