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Homework Help: Maths help!

  1. Feb 5, 2005 #1
    How do you change this [tex]A = 720 \pi - \frac{4}{3} \pi r^3 + 3 \pi r^2[/tex] into [tex]A = 5 \pi (\frac{144}{r} + \frac{r^2}{3})[/tex]?
     
  2. jcsd
  3. Feb 5, 2005 #2

    Galileo

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    Uuhm. You don't?
     
  4. Feb 5, 2005 #3
    [tex]A = 720 \pi - \frac{4}{3} \pi r^3 + 3 \pi r^2[/tex]

    [tex]A = \pi (720 - \frac{4}{3} r^3 + 3 r^2)[/tex]

    [tex]A = \pi (\frac{2160}{3} - \frac{4}{3} r^3 + \frac{9}{3} r^2)[/tex]

    [tex]A = \pi (\frac{2160 - 4 r^3 + 9 r^2}{3})[/tex]

    [tex]A = \frac{1}{3}\pi (2160 - 4 r^3 + 9 r^2)[/tex]

    After this I have no idea at the present moment.

    The Bob (2004 ©)
     
  5. Feb 5, 2005 #4

    dextercioby

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    The two expressions are not equal,which means you cannot find a chain of correct equalities which would bring one into another.

    Daniel.
     
  6. Feb 5, 2005 #5
    Are you sure you typed them in right? They don't seem equivalent to me either. :confused:
     
  7. Feb 5, 2005 #6
    Oh good. I thought it was just me being stupid.

    The Bob (2004 ©)
     
  8. Feb 5, 2005 #7
    You guys are right! Their not equal. After redoing it two times, I got the right equation. :grumpy: Here it is: [tex]A = \frac{720 \pi - \frac{4}{3} \pi r^3}{r} + 3 \pi r^2[/tex]. Thanks for the help guys! :smile:
     
  9. Feb 5, 2005 #8
    So do you still need help, or you know how to change one into the other?
     
  10. Feb 6, 2005 #9
    No I don't need help.
     
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