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 - Section 4.4 - Concavity and Curve Sketching - Exercises 4.4 - Page 213: 67

Answer

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Work Step by Step

Step 1. Given the first derivative $y'=x^{-2/3}(x-1)=x^{1/3}-x^{-2/3}$, we have $y''=\frac{1}{3}x^{-2/3}+\frac{2}{3}x^{-5/3}=\frac{1}{3}x^{-5/3}(x+2)$ Step 2. Possible critical points are at $x=0,1$. Determine the increasing ($y'\gt0$) and decreasing ($y'\lt0$) regions by signs of $y'$: $..(-)..(0)..(-)..(1)..(+)..$; thus the function decreases on $(-\infty,0),(0,1)$ and increases on $(1,\infty)$. A local and absolute minimum can be found at $x=1$. There is no local or absolute maximum. Step 3. Use signs of $y''$ to determine concavity and possible inflection points at $x=-2,0$: $..(+)..(-2)..(-)..(0)..(+)..$. Thus, the function is concave up on $(-\infty,-2)$,$(0,\infty)$ and concave down on $(-2,0)$. Thus $x=-2$ and $x=0$ are inflection points. Step 4. Sketch the function based on the above results as shown.
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