- #1
songoku
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Homework Statement
Given that a solid cylinder has a fixed volume V, prove that its total surface area S is minimum when its height and base diameter are equal.
Homework Equations
derivative
The Attempt at a Solution
I am able to prove that question.
[tex]V=\pi r^2 h[/tex]
[tex]h=\frac{V}{\pi r^2}[/tex]
So, to get minimum surface area:
[tex]\frac{dS}{dr}=0[/tex]
[tex]\frac{d}{dr}(2\pi r h + 2 \pi r^2)=0[/tex]
[tex]\frac{d}{dr}(2\pi r \frac{V}{\pi r^2} + 2 \pi r^2)=0[/tex]
[tex]\frac{d}{dr}(2\frac{V}{r}+2 \pi r^2)=0[/tex]
[tex]-2\frac{V}{r^2}+4\pi r=0[/tex]
[tex]2\frac{V}{r^2}=4\pi r[/tex]
[tex]2\frac{\pi r^2 h}{r^2}=4\pi r[/tex]
[tex]h=d\; \text{(Shown)}[/tex]
So,with h = d, the minimum surface area is :
[tex]S=2\pi r (2r) + 2\pi r^2[/tex]
[tex]S=6\pi r^2[/tex]
What I want to ask is : how about if the question asks to find the maximum surface area?
I think to find the maximum value, we also set [tex]\frac{dS}{dr}=0[/tex]. From my work, I don't see any ways to find the maximum value...
Thanks