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Homework Help: Optimization: Open Box help

  1. Nov 22, 2009 #1
    1. The problem statement, all variables and given/known data
    http://img7.imageshack.us/img7/1826/43544187.jpg [Broken]

    2. Relevant equations

    3. The attempt at a solution
    whats wrong with my answers? everything looks right to me... :S
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 23, 2009 #2


    User Avatar
    Science Advisor

    The only thing wrong that I see is that you haven't answered the question. You said that the volume will be minimized. You have given x, but you haven't found y and have not said what the dimensions are.
  4. Nov 24, 2009 #3

    the thing is, i only have to choose the right answer from those drop down menu boxes..
    so i must of chosen something wrong.. but what? i dont see any mistakes
  5. Nov 24, 2009 #4
    okay my derivatives look fine.. when f'(x)=0 x=17.1
    f''(x) > 0 for x>0.. thats right because if plugging in a negative number i will get f''(x) = -..

    so whats wrong?
  6. Nov 24, 2009 #5
    can someone please help me?
  7. Nov 25, 2009 #6
    why are u guys ignoring this post? is it something that i said?
    for the last part where it says it will be relative min, when x=___
    would it be -21.5446 ?
    i got it by getting the second derivative equal to 0
  8. Nov 25, 2009 #7


    Staff: Mentor

    One of your entry boxes says "This implies that the surface area is given in S only..."
    Except for this, everything else it looks fine.

    Here's a tip you might consider. Many or most of the problems you have posted have oddball numbers such as a volume of V = 2500.1055 cm^3.
    I did all of my calculations using V, and replaced V only in the very last step. This saved my from writing 2500.1055 a bunch of times.

    For example, A = x^2 + 4V/x. It's easy to get dA/dx = 2x -4V/x^2. Rewriting this as dA/dx = 2x -4Vx-2, it's easy to get the second derivative and verify that it's positive for all x > 0.
  9. Nov 25, 2009 #8

    i did it this way.. i substituted V only at the very end and i got the same answers..
    and about the "This implies that the surface area is given in S only..." yeah i didnt read it carefully but still. now i got it tnx!
    Last edited: Nov 25, 2009
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