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Homework Help: Derivate question

  1. Oct 9, 2011 #1
    Example Problem: Finding dy/dx for cos(2x).

    knowing that the derivative of cos(x) is -sin(x)...

    thus my thought is... when differentiating cos(2x)... why is the derivative not -sin(2x)?

    I wasn't aware that I would have to treat that as a composite function.....
  2. jcsd
  3. Oct 9, 2011 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Have you learned the chain rule yet?
  4. Oct 9, 2011 #3
    Yes I have... But I thought I didn't think I had to use the chain rule on this function.
  5. Oct 9, 2011 #4
    You have to use the chain rule. You are right in which cos(x) is -sin(x) but you also have 2x which the derivative is 2 so the final answer is -2sin(x)

    Hope it make things more clear for you.
  6. Oct 9, 2011 #5


    Staff: Mentor

    No, cos(x) IS NOT -sin(x), but d/dx(cos(x)) = -sin(x).
  7. Oct 9, 2011 #6
    You knew what I meant. :D
  8. Oct 9, 2011 #7


    Staff: Mentor

    Yes, I knew what you meant, so you should work a little harder to say what you mean.
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