Calculus 10th Edition

Published by Brooks Cole
ISBN 10: 1-28505-709-0
ISBN 13: 978-1-28505-709-5

Chapter 3 - Applications of Differentiation - 3.6 Exercises - Page 212: 20

Answer

Graph

Work Step by Step

$$\eqalign{ & y = - \frac{1}{3}\left( {{x^3} - 3x + 2} \right) \cr & {\text{Domain: }}\left( { - \infty ,\infty } \right) \cr & {\text{Find the }}y{\text{ intercept, let }}x = 0 \cr & y = - \frac{1}{3}\left( {{{\left( 0 \right)}^3} - 3\left( 0 \right) + 2} \right) \cr & y = - \frac{2}{3} \cr & y{\text{ - intercept }}\left( {0, - \frac{2}{3}} \right) \cr & {\text{Find the }}x{\text{ intercept, let }}y = 0 \cr & 0 = - \frac{1}{3}\left( {{x^3} - 3x + 2} \right) \cr & {\text{Solving by a calculator}} \cr & x = - 2,{\text{ }}x = 1 \cr & x{\text{ - intercepts }}\left( { - 2,0} \right),\left( {1,0} \right) \cr & \cr & *{\text{Find the extrema}} \cr & {\text{Differentiate}} \cr & y' = \frac{d}{{dx}}\left[ { - \frac{1}{3}\left( {{x^3} - 3x + 2} \right)} \right] \cr & y' = \frac{d}{{dx}}\left[ { - \frac{1}{3}{x^3} + x - \frac{2}{3}} \right] \cr & y' = - {x^2} + 1 \cr & {\text{Let }}y' = 0{\text{ to find critical points}} \cr & - {x^2} + 1 = 0 \cr & x = \pm 1 \cr & *{\text{Find the second derivative}} \cr & y'' = \frac{d}{{dx}}\left[ { - {x^2} + 1} \right] \cr & y'' = - 2x \cr & {\text{Evaluate }}y''\left( x \right){\text{ at }}x = \pm 1 \cr & y''\left( { - 1} \right) = 2 > 0,{\text{ relative minimum at }}x = - 1 \cr & f\left( { - 1} \right) = - \frac{4}{3} \to {\text{Relative minimum at }}\left( { - 1, - \frac{4}{3}} \right) \cr & y''\left( 1 \right) = - 2 < 0,{\text{ relative maximum at }}x = 1 \cr & f\left( 1 \right) = 0 \to {\text{Relative maximum at }}\left( {1,0} \right) \cr & \cr & {\text{Let }}y''\left( x \right) = 0 \cr & - 2x = 0 \cr & x = 0 \cr & x = \pm \sqrt {\frac{{27}}{2}} \left( {{\text{These vales are not in the domain}}} \right),{\text{ }}x = 0 \cr & x = 0 \cr & {\text{Inflection point }}\left( {0,f\left( 0 \right)} \right) \cr & y = - \frac{1}{3}\left( {{{\left( 0 \right)}^3} - 3\left( 0 \right) + 2} \right) \cr & y = - \frac{2}{3} \cr & {\text{Inflection point}} \to \left( {0, - \frac{2}{3}} \right) \cr & \cr & {\text{*Calculate the asymptotes}} \cr & {\text{No vertical asymptotes, the denominator is 1}}{\text{.}} \cr & \mathop {\lim }\limits_{x \to \infty } \left( {2 - x - {x^3}} \right) = - \infty \cr & \mathop {\lim }\limits_{x \to - \infty } \left( {2 - x - {x^3}} \right) = + \infty \cr & {\text{No horizontal asymptotes}} \cr & \cr & {\text{Graph}} \cr} $$
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