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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!

Warren

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

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