Thomas' Calculus 13th Edition

Published by Pearson
ISBN 10: 0-32187-896-5
ISBN 13: 978-0-32187-896-0

Chapter 4: Applications of Derivatives - Practice Exercises - Page 244: 55

Answer

height $h=2$, radius $r=\sqrt 2$

Work Step by Step

Step 1. We illustrate the problem with the shown figure, where $R=\sqrt 3$ is the radius of the sphere and $r, h$ are the radius and height of the cylinder. Step 2. Using Pythagorean Theorem, we have $R^2=r^2+(\frac{h}{2})^2$, which gives $r^2=3-(\frac{h}{2})^2$ Step 3. The volume of the cylinder is given by $V=\pi r^2 h=\pi h(3-(\frac{h}{2})^2)=\pi(3h-\frac{1}{4}h^3)$ Step 4. To maximize $V$, let $V'=0$, we have $3-\frac{3}{4}h^2=0$, which gives $h=2$ (positive answer only). Thus $r^2=3-(\frac{h}{2})^2=2$ and $r=\sqrt 2$ Step 5. Check $V''=-(\frac{3}{2})h\lt0$; we confirm the above dimensions give a maximum volume.
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