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Homework Help: Rate of Change

  1. Apr 21, 2010 #1
    1. The problem statement, all variables and given/known data

    attachment.php?attachmentid=25294&stc=1&d=1271861458.jpg

    2. Relevant equations



    3. The attempt at a solution

    v=1/3 pi r^2 h
    dv/dt = 1/3 pi (2rh dr/dt + r^2 dh/dt)
    now i sub r=15/3 and h=3 .
    Now i express dr/dt=dr/dh x dh/dt
    and find dr/dh .
    But it fails
     

    Attached Files:

  2. jcsd
  3. Apr 21, 2010 #2

    sylas

    User Avatar
    Science Advisor

    The equation for v looks okay, as long as h is <= 4.

    The substitution r = 15/3 is wrong.

    You should be able to express r in terms of h. What is the relation between r and h? Note that if you use this, you can get just get rid of r altogether, which will be nice.

    Cheers -- sylas
     
  4. Apr 21, 2010 #3
    could it be r=h tan (a) ? thanks btw =D
     
  5. Apr 21, 2010 #4

    sylas

    User Avatar
    Science Advisor

    No.

    Look again at the figure. Draw some water in it. Mark h and r for the given volume.
     
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