University Calculus: Early Transcendentals (3rd Edition)

Published by Pearson
ISBN 10: 0321999584
ISBN 13: 978-0-32199-958-0

Chapter 4 - Section 4.1 - Extreme Values of Functions - Exercises - Page 216: 66

Answer

At $x=e^{-1/2}$, the function has an absolute minimum of $\displaystyle \frac{-1}{2e}$

Work Step by Step

The domain is $(0,+\infty)$. The graph crosses the x-axis at $x=1.$ When $x\rightarrow 0^{+},$ observing a few function values $\left[\begin{array}{l} f(0.1)\approx-0.023\\ f(0.001)\approx-6.9\times 10^{-6} \end{array}\right],$ we note that the graph is below the x-axis, approaching the origin. $\displaystyle \lim_{x\rightarrow\infty}f(x)=\infty,\qquad $(there are no absolute maxima) Find critical points: $f'(x)=[x^{2}]'\displaystyle \ln x+x^{2}\cdot[\ln x]'=2x\ln x+x^{2}\cdot\frac{1}{x}=x(2\ln x+1)$ $f'(x)=0$ when $\left[\begin{array}{l} 2\ln x+1=0\\ 2\ln x=-1\\ \ln x=-\frac{1}{2}\\ x=e^{-1/2}\approx 0.60653 \end{array}\right]$ Evaluate at $x=e^{-1/2},\quad $ $f(e^{-1/2})=(e^{-1/2})^{2}\displaystyle \cdot(-\frac{1}{2})=\frac{-1}{2e}\approx-0.1839$ At $x=e^{-1/2}$, the function has an absolute minimum of $\displaystyle \frac{-1}{2e}$
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