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Derivative Chain Rule Problem

  1. Oct 15, 2011 #1
    I did a derivative problem, but my book says that my answer is wrong.

    f(x)=x2(x-2)4

    I didn't see much use in the chain rule so I used the product rule.

    x2(4(x-2)3) + (x-2)4(2x)
    =4x2(x-2)3 + 2x(x-2)4

    The book says that instead of this, the answer is ...
    x2(4(x-2)3(1)) + (x-2)4(2x) = 2x(x-2)3(3x-2)

    So I guess where I'm really confused is in the manner with which the book simplified.
     
  2. jcsd
  3. Oct 15, 2011 #2

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Well, you did use the chain rule when you said that the derivative of (x- 2)4 is 4(x- 2)3 (because the derivative of x- 2 is 1).

    If you factor out 2x(x- 2)3 to get f'= 2x(x- 2)3(2x+ (x- 2))= 2x(x- 2)(3x- 2), exactly what your book says. Your answer and your book's answer are the same.

     
  4. Oct 15, 2011 #3
    Awesome thanks, I get it now. And I also now understand how ridiculous my book is in its mindless factoring.
     
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