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Homework Help: F'(x) = 0 Trig ID Problems

  1. Jul 14, 2011 #1

    This is just a part of a calculus 1 problem. It occurs while attempting to make f'(x) = 0

    f'(x) = cosX - 1/6cos3X

    Now to make this zero I quickly note that in the domain of (0-2pi) cos=0 at pi/2 and 3pi/2 so the expression equals zero already! I have been informed by my instructor I need to use a series of identities to achieve a final answer close to

    cosX[5/6 +2/3sin^2x]

    While it may very well include the cos zero I outlined above (and in fact insists on it!) it is now more complicated...... sigh....

    Thing is I would never make this connection on exam day without seeing what is going on. Tutors give me that look like it's obvious as does my instructor but I just don't see how or why I should stretch it and why if so stop at this point as I could continue....

    ANY help advise greatly appreciated!

  2. jcsd
  3. Jul 14, 2011 #2


    Staff: Mentor

    More precisely, the derivative is zero at those two values.
    I'm confused. Are you supposed to rewrite your derivative as above? If so, start by replacing cos(3x) with cos(2x + x) and using the addition formula.
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