Calculus with Applications (10th Edition)

Published by Pearson
ISBN 10: 0321749006
ISBN 13: 978-0-32174-900-0

Chapter 5 - Graphs and the Derivative - Chapter Review - Review Exercises - Page 297: 31

Answer

\[\begin{align} & \text{Relative maximum at }x=\frac{1-\sqrt{5}}{2} \\ & \text{Relative minimum at }x=\frac{1+\sqrt{5}}{2} \\ \end{align}\]

Work Step by Step

\[\begin{align} & f\left( x \right)=\frac{x{{e}^{x}}}{x-1} \\ & \text{Calculating the first derivative} \\ & f'\left( x \right)=\frac{d}{dx}\left[ \frac{x{{e}^{x}}}{x-1} \right] \\ & f'\left( x \right)=\frac{\left( x-1 \right)\frac{d}{dx}\left[ x{{e}^{x}} \right]-x{{e}^{x}}\frac{d}{dx}\left[ x-1 \right]}{{{\left( x-1 \right)}^{2}}} \\ & f'\left( x \right)=\frac{\left( x-1 \right)\left( x{{e}^{x}}+{{e}^{x}} \right)-x{{e}^{x}}}{{{\left( x-1 \right)}^{2}}} \\ & f'\left( x \right)=\frac{{{x}^{2}}{{e}^{x}}+x{{e}^{x}}-x{{e}^{x}}-{{e}^{x}}-x{{e}^{x}}}{{{\left( x-1 \right)}^{2}}} \\ & f'\left( x \right)=\frac{{{x}^{2}}{{e}^{x}}-x{{e}^{x}}-{{e}^{x}}}{{{\left( x-1 \right)}^{2}}} \\ & \text{Calculating the critical points, set }f'\left( x \right)=0 \\ & f'\left( x \right)={{x}^{2}}{{e}^{x}}-x{{e}^{x}}-{{e}^{x}} \\ & {{e}^{x}}\left( {{x}^{2}}-x-1 \right)=0 \\ & {{x}^{2}}-x-1=0 \\ & \text{By the quadratic formula} \\ & {{x}_{1}}=\frac{1+\sqrt{5}}{2},\text{ }{{x}_{2}}=\frac{1-\sqrt{5}}{2} \\ & \text{And the derivative is not defined for }x=1 \\ & \text{Set the intervals }\left( -\infty ,\frac{1-\sqrt{5}}{2} \right),\left( \frac{1-\sqrt{5}}{2},1 \right) \\ & \text{and}\left( 1,\frac{1+\sqrt{5}}{2} \right),\left( \frac{1+\sqrt{5}}{2},\infty \right) \\ & \text{Making a table of values} \\ & \begin{matrix} \text{Interval} & \text{Test Value} & \text{Sign of }f'\left( x \right) & \text{Conclusion} \\ \left( -\infty ,\frac{1-\sqrt{5}}{2} \right) & x=-2 & + & \text{Increasing} \\ \left( \frac{1-\sqrt{5}}{2},1 \right) & x=0 & - & \text{Decreasing} \\ \left( 1,\frac{1+\sqrt{5}}{2} \right) & x=\frac{3}{2} & - & \text{Decreasing} \\ \left( \frac{1+\sqrt{5}}{2},\infty \right) & x=2 & + & \text{Increasing} \\ \end{matrix} \\ & \text{Therefore,} \\ & \text{Relative maximum at }x=\frac{1-\sqrt{5}}{2} \\ & \text{Relative minimum at }x=\frac{1+\sqrt{5}}{2} \\ \end{align}\]
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