Optimization - Find dimension of a cup that uses least amount of paper

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Homework Help Overview

The problem involves optimizing the dimensions of a cone-shaped paper drinking cup to minimize the surface area while maintaining a fixed volume of 30 cm3. The subject area includes optimization techniques in calculus.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss deriving the height from the volume equation and substituting it into the surface area formula. There are attempts to differentiate the surface area with respect to the radius and set the derivative to zero to find critical points. Some participants question the correctness of each other's derivatives.

Discussion Status

The discussion is ongoing, with participants sharing their attempts and derivatives. There is no explicit consensus on the correct approach or answers, as discrepancies in calculations have been noted.

Contextual Notes

Participants are working under the constraint of a fixed volume for the cup and are exploring the relationship between the radius and height in the context of minimizing surface area. There is an emphasis on showing work to identify errors in reasoning or calculation.

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Homework Statement


A cone-shaped paper drinking cup is to be made to hold 30 cm3 of water. Find the height and radius of the cup that will use the smallest amount of paper.


Homework Equations


volume of a cone (1/3)(pi)(r^2)(h) = 30
SA of a cone pi(r)[sqrt(r^2 + h^2)]


The Attempt at a Solution


solve volume for h
plug h into SA
derive SA
calculate r
solve for h

I'm understanding how to do it, I just can't get the right answers.
 
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Show your work, so we can know where did you go wrong.
 


h = 90/(pir^2)

derivative of SA = 2pi*pir^2*r^2+90pi, denominator not needed, setting equal to zero.
 


I got a different derivative (even after clearing out some denominators).
 

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