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Calculate f'(x) given d/dx(f(3x^4))=6x^4?

  1. Sep 21, 2010 #1
    I am told that d/dx(f(3x^4))=6x^4. I need to calculate f ' (x). i have tried like 3 different methods and have no idea how to do this.
    Help please!
     
  2. jcsd
  3. Sep 21, 2010 #2

    Office_Shredder

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    Can you find f'(3x4)?
     
  4. Sep 22, 2010 #3

    HallsofIvy

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    By the chain rule,
    [tex]\frac{df(3x^4)}{dx}= f'(3x^4)(12x^3)[/tex]

    So you must have [itex]f'(3x^4)(12x^3)= 6x^4[/itex].

    That lets you find [itex]f'(3x^4)[/itex] per Office Shredder's question. Now, if you let [itex]u= 3x^4[/itex], you can find f'(u).
     
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