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Homework Help: Derivatives. Product rule with 3 products

  1. Nov 21, 2007 #1
    1. The problem statement, all variables and given/known data

    If f(x) = (3 x )(sin x) (cos x), find f'( x ).

    A question I have is , is there anything special to do when you have 3 products instead of 2

    3. The attempt at a solution

    Well I used the product rule as if am multipling

    (3xsinx) (cosx)

    but that doesn't seem to get me the answer or maybe Its something about my answer because I put answers in a computer so sometimes its the notation.

    here is my final answer
  2. jcsd
  3. Nov 21, 2007 #2


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    Homework Helper

    well for 3 products...just take take the product of 2 terms and multiply by the differential of the of the 3rd term...if you dont get it

  4. Nov 21, 2007 #3
    K managed to get it off of that thx
  5. Aug 16, 2011 #4
    although the equation above is impressive and simple. I hate to learn yet another differentiation rule. your initial approach is correct.

    (3xsinx) (cosx) = [(3x sinx) (-sin x)]+ [first ' * (cosx)]

    the same old (first * second ') + (first ' * second) product rule.

    now take derivative of (3x * sin x) with the product rule and plug it in where first' goes.

    your computer probably does a better job of simplification than u :)
    Last edited: Aug 16, 2011
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