Math Problem Statement

You are designing a 32 cm^3 right circular cylindrical can whose manufacture will take waste into account. There is no waste in cutting the aluminum for the side, but the top and bottom of radius r will be cut from squares that measure 4r units on a side. The total amount of aluminum used up by the can will therefore be A = 32r^2 + 2πrh, where h is the height of the right circular cylindrical can. What is the ratio of h to r for the most economical can?

Solution

Let's analyze the problem step by step.

Given:

  1. Volume of the cylinder: V=πr2h=32cm3V = \pi r^2 h = 32 \, \text{cm}^3.
  2. Surface area to minimize: A=32r2+2πrhA = 32r^2 + 2\pi r h, where rr is the radius and hh is the height.

Steps to solve:

  1. From the volume equation πr2h=32\pi r^2 h = 32, solve for hh: h=32πr2.h = \frac{32}{\pi r^2}.

  2. Substitute hh into the surface area equation AA: A=32r2+2πr(32πr2).A = 32r^2 + 2\pi r \left(\frac{32}{\pi r^2}\right). Simplify: A=32r2+64r.A = 32r^2 + \frac{64}{r}.

  3. Differentiate AA with respect to rr to find the critical points: dAdr=64r64r2.\frac{dA}{dr} = 64r - \frac{64}{r^2}.

  4. Set dAdr=0\frac{dA}{dr} = 0 to minimize AA: 64r64r2=0.64r - \frac{64}{r^2} = 0. Multiply through by r2r^2 to eliminate the denominator: 64r3=64    r3=1    r=1.64r^3 = 64 \quad \implies \quad r^3 = 1 \quad \implies \quad r = 1.

  5. Find the corresponding hh using h=32πr2h = \frac{32}{\pi r^2}: h=32π(1)2=32π.h = \frac{32}{\pi (1)^2} = \frac{32}{\pi}.

  6. Calculate the ratio hr\frac{h}{r}: hr=32π1=32π.\frac{h}{r} = \frac{\frac{32}{\pi}}{1} = \frac{32}{\pi}.

Final Answer:

The ratio of hh to rr for the most economical can is: 32π\boxed{\frac{32}{\pi}}

Let me know if you want additional clarification or further steps! Here are some related questions to expand this topic:

  1. How would the solution change if the volume of the cylinder were different?
  2. Can you explain why minimizing surface area is equivalent to minimizing material waste in this context?
  3. What role does differentiation play in solving optimization problems like this?
  4. How does the ratio h/r=32πh/r = \frac{32}{\pi} compare to the dimensions of a sphere with the same volume?
  5. Could you explore how manufacturing constraints (e.g., material thickness) might affect the optimization problem?

Tip: Always double-check the units in optimization problems, as they often hint at what the variables represent!

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Math Problem Analysis

Mathematical Concepts

Optimization
Differentiation
Volume and Surface Area of Cylinders

Formulas

Volume of a cylinder: V = πr^2h
Surface area to minimize: A = 32r^2 + 2πrh

Theorems

Optimization using differentiation

Suitable Grade Level

Grades 10-12