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Is there an identity for the cos( 1/3 x ) ?

  1. Aug 25, 2012 #1
    It seems that this term comes up in solving the cubic equation. While there is the identity for the half-angle, there doesn't seem to be one for third-angle.
  2. jcsd
  3. Aug 25, 2012 #2


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  4. Aug 26, 2012 #3
    Hi !

    The identity involves complex terms :

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    Last edited: Aug 26, 2012
  5. Aug 28, 2012 #4
    OK, I understand the general form. I didn't think to use an integer fraction in lieu of an integer. (Actually, it appears that *any* number could be used there.)
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