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Homework Help: Finding Volume using integration

  1. Jul 3, 2008 #1
    1. The problem statement, all variables and given/known data
    Find the volume of the solid formed by rotating the region enclosed by:
    (e^(1x)+2)/y=0/x=0/x=.9
    about the y axis


    2. Relevant equations
    Probably disk method i would assume:
    V=pi*int((f(x)^2) dx from bounds a to b


    3. The attempt at a solution

    V= pi*int(e^(1x)+2)^2) a=0 b=.9
    v=pi*int(e^(2x)+4) a=0 b=.9
    v=pi*(1/2e^(2x)+4x)
    v=pi*(1/2e^(2(.9))-(1/2e^0)+(4(.9)
    v=pi*(3.025)-(1/2)+3.6
    v=6.125pi

    But my answer is not correct.
     
    Last edited: Jul 3, 2008
  2. jcsd
  3. Jul 3, 2008 #2

    tiny-tim

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    Hi Mcbrown108! :smile:

    Isn't that for rotation about the x-axis? :cry:
     
  4. Jul 3, 2008 #3
    Oh yeah. so then would i change my bounds to a=-.9 b=.9
     
  5. Jul 3, 2008 #4
    that doesnt seem to work either
     
  6. Jul 3, 2008 #5

    tiny-tim

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    It's not your bounds that are wrong … it's your discs.

    Your discs should be "horizontal" discs, centred on the y-axis. :smile:
     
  7. Jul 3, 2008 #6
    So i should plug in the given x's to get y's for my bounds?
     
  8. Jul 3, 2008 #7

    tiny-tim

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    I've no idea what that means, but I'm going to guess the answer is … "YES!!"

    Go for it! :smile:
     
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