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How is my integration wrong? X(

  1. Jan 27, 2013 #1
    My set-up for the integral is right:

    2[itex]\pi[/itex]∫0 to 4 of (x)(√(4x) - (2x - 4))dx

    = 2[itex]\pi[/itex]∫(2x3/2 - 2x2 + 4)dx

    When I Wolfram Alpha the answer, it is 448[itex]\pi[/itex]/15.
    The computer HW system counts that answer as correct.

    But when I try to take the integral, it's wrong! :(

    I did this...:

    2[itex]\pi[/itex](2x5/2/5/2 - 2(x3)/3 + 4x2/4] |0 to 4

    = 29.86[itex]\pi[/itex]

    ...This is not right. But what's the matter with my integration?

    Thanks SO much for the help! :)
     
  2. jcsd
  3. Jan 27, 2013 #2

    Dick

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    Where did 4x^2/4 come from?
     
  4. Jan 27, 2013 #3
    Oh, sorry. I wrote down (4x^2)/2 on my paper but typed it wrong!

    I meant (4x^2)/2
     
  5. Jan 27, 2013 #4

    Curious3141

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    448/15 = 29.8666666...

    I'm guessing the system wants the exact answer, not a rounded off decimal.
     
  6. Jan 27, 2013 #5

    Dick

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    Ok. 448/15=29.8666... What do you think is wrong??
     
  7. Jan 27, 2013 #6

    Dick

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    That's a good guess.
     
  8. Jan 27, 2013 #7
    Yeah, I guess it was the rounding thing! Man! :/

    Thank you SO much for helping! Simple problem! UGH! =_=
     
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