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Homework Help: Derivatives, chain rule 3

  1. Nov 9, 2013 #1
    1. The problem statement, all variables and given/known data

    Find the derivative of y=cos(a3+x3)

    2. Relevant equations

    Chain rule

    3. The attempt at a solution

    y=cosu

    [itex]\frac{dy}{du}[/itex]
    = -sinu

    u=a3+x3

    [itex]\frac{du}{dx}[/itex]
    = 3a2+3x2


    [itex]\frac{dy}{dx}[/itex] = -3sin(a3+x3)(a2+x2).

    The answer is supposed to be -3x2sin(a3+x3). What did I do wrong?

    Any help is much appreciated.
     
  2. jcsd
  3. Nov 9, 2013 #2

    LCKurtz

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    ##a## is constant so the derivative of ##a^3## is ##0##.

    Please do not insert SIZE commands in your posts. It makes them very hard to follow when quoting them, aside from violating forum policy.
     
  4. Nov 9, 2013 #3
    Okay, thanks!

    I was thinking maybe that was my error, but I wasn't 100% sure.

    And I didn't know about the SIZE commands rule, so I'll stay away from them from now on. I just thought they'd be easier to read since I don't know how to make the equations like yours.
     
  5. Nov 9, 2013 #4

    LCKurtz

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