Calculus: Early Transcendentals (2nd Edition)

Published by Pearson
ISBN 10: 0321947347
ISBN 13: 978-0-32194-734-5

Chapter 4 - Applications of the Derivative - 4.3 Graphing Functions - 4.3 Exercises - Page 268: 33

Answer

$$\eqalign{ & {\text{Domain }}\left( {0,\infty } \right) \cr & {\text{No }}y{\text{ - intercepts}} \cr & x{\text{ - intercepts: }}\left( {1,0} \right) \cr & {\text{Local maximum }}\left( {\frac{1}{e}, - \frac{1}{e}} \right) \cr & {\text{No inflection points}} \cr} $$

Work Step by Step

$$\eqalign{ & f\left( x \right) = x\ln x \cr & {\text{Domain }}\left( {0,\infty } \right) \cr & {\text{Find the }}y{\text{ intercept}}{\text{, let }}x = 0,{\text{ }}x = 0{\text{ is not in the domain}}{\text{.}} \cr & {\text{There are no }}y{\text{ - intercepts }} \cr & {\text{Find the }}x{\text{ intercept}}{\text{, let }}f\left( x \right) = 0 \cr & x\ln x = 0 \cr & x = 0{\text{ and }}x = 1 \cr & x = 0{\text{ is not in the domain}}{\text{, then }}x = 1 \cr & x{\text{ - intercept: }}\left( {1,0} \right) \cr & \cr & *{\text{Find the extrema}} \cr & {\text{Differentiate}} \cr & f'\left( x \right) = \frac{d}{{dx}}\left[ {x\ln x} \right] \cr & f'\left( x \right) = \ln x + x\left( {\frac{1}{x}} \right) \cr & f'\left( x \right) = \ln x + 1 \cr & {\text{Let }}f'\left( x \right) = 0{\text{ to find critical points}} \cr & \ln x + 1 = 0 \cr & \ln x = - 1 \cr & x = {e^{ - 1}} \cr & x = \frac{1}{e} \cr & \cr & *{\text{Find the second derivative}} \cr & f''\left( x \right) = \frac{d}{{dx}}\left[ {\ln x + 1} \right] \cr & f''\left( x \right) = \frac{1}{x} \cr & \cr & {\text{Evaluate }}f''\left( x \right){\text{ at the critical point }}x = \frac{1}{e} \cr & f''\left( {\frac{1}{e}} \right) = \frac{1}{{1/e}} = e> 0,{\text{ then}} \cr & {\text{There is a local minimum at }}\left( {\frac{1}{e},f\left( {\frac{1}{e}} \right)} \right) \cr & f\left( {\frac{1}{e}} \right) = \left( {\frac{1}{e}} \right)\ln \left( {\frac{1}{e}} \right) = - \frac{1}{e} \cr & \to {\text{local minimum at }}\left( {\frac{1}{e}, - \frac{1}{e}} \right) \cr & \cr & {\text{*Find the Inflection points}}{\text{, set }}f''\left( x \right) = 0 \cr & \frac{1}{x} = 0 \cr & {\text{No solution}}{\text{, then there are no inflection points}}{\text{.}} \cr & \cr & {\text{Graph}} \cr} $$
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