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Homework Help: Applied problem using absolute extrema

  1. Apr 8, 2010 #1
    Hi, I have a problem solving this question, I don't know if I am on the correct track for solving this. Suppose that the sum of the surfaces of a sphere and a cube is constant. Show that the sum of their volumes is smallest when the diameter of the sphere is equal to the length of an edge of the cube.
    Let x = length of an edge of the cube. r= radius of the sphere and V = total volume. S= total surface area. Then, [tex] V=\frac{4}{3} \pi r^3 + x^3 [/tex]
    [tex] S=4\pi r^2 + 6x^2 [/tex] => [tex] x^2=\frac{S-4\pi r^2}{6} [/tex] =>
    [tex] x^6=(\frac{S}{6}-\frac{2}{3} \pi r^2)^3[/tex]
    => [tex] x^3=\sqrt{(\frac{S}{6}-\frac{2}{3} \pi r^2)^3}[/tex]. It follows from this that [tex] V=\frac{4}{3} \pi r^3 + \sqrt{(\frac{S}{6}-\frac{2}{3} \pi r^2)^3}[/tex]. This looks like an unyieldy equation. I have yet to differentiate V w.r.t. r, and put V'(r)=0. Do I expand it using the Binomial Theorem or am I totally wrong in my approach to the question? Thanks for the help.
  2. jcsd
  3. Apr 9, 2010 #2
    Use implicit differentiation & don't bother to find x in terms of r.
  4. Apr 10, 2010 #3
    Ok, I used implicit differentiation to get [tex] \frac{dV}{dr}=4 \pi r^2 + 3x^2 \frac{dx}{dr} = 0 [/tex] I cannot see anything in the question that tells me the value of dx/dr. So can someone give a hint as to where to go from here. Thanks very much.
  5. Apr 11, 2010 #4


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    Hint: :smile:
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