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Homework Help: What am I dong wrong here?

  1. Apr 17, 2016 #1
    1. The problem statement, all variables and given/known data

    If y' = 6x(x2+2)2 and y = 25 when x = 1, what is the value of y when x = -1?

    2. Relevant equations N/A

    3. The attempt at a solution


    ∫6x(x2+2)2dx = 2x(x2+2)3

    (12 + 3)3 + 2(1) + c = 25

    1 + 2 = 3 and 2(1) = 2 and 33 = 27, so 27*2 + c = 25 so 54 + c = 25, therefore c = 25-54 = -29

    Now substitute x = -1

    (-1) squared is 1 and 2(-1) = -2, so 27*-2 = -54, so -54 - 29 = y = -83

    WileyPlus tells me it's wrong.

    What is wrong with this?
     
  2. jcsd
  3. Apr 17, 2016 #2

    SammyS

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    You need a constant of integration in that result.

    Also, you made a mistake in your integration.

    What is the derivative of y = 2x(x2+2)3 + C ?

     
  4. Apr 17, 2016 #3
    OH! *smacks self* Never mind...

    And I just forgot to type in the + c; you'll see in my calculations that it appears there, sorry.

    EDIT: Thanks, BTW, like a LOT!
     
  5. Apr 17, 2016 #4
    Thanks a lot, SammyS!
     
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