Calculus with Applications (10th Edition)

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

Chapter 5 - Graphs and the Derivative - 5.3 Higher Derivatives, Concavity, and the Second Derivative Test - 5.3 Exercises - Page 284: 38

Answer

$$ f(x)=\frac{-2}{x+1} $$ $f(x)$ concave upward on $$(-\infty,-1 ) $$. $f(x)$ concave downward on $$(-1 , \infty).$$ No Inflection point .

Work Step by Step

$$ f(x)=\frac{-2}{x+1} $$ By the quotient rule,we can find the first derivative as follows $$ \begin{aligned} f^{\prime}(x) &=\frac{0+2(1)}{(x+1)^{2}}\\ &=\frac{2}{(x+1)^{2}}, \end{aligned} $$ and the second derivative is $$ \begin{aligned} f^{\prime\prime}(x) &=\frac{0-2(2)(x+1)(1)}{(x+1)^{4}}\\ &=\frac{-4}{(x+1)^{3}}. \end{aligned} $$ A function $f$ is concave upward on an interval if $f^{\prime\prime}(x) \gt 0$ for all $x$ in the interval, so $$ \begin{aligned} f^{\prime\prime}(x) & =\frac{-4}{(x+1)^{3}} \gt 0 . \end{aligned} $$ when $$ \begin{aligned} (x+1)^{3} &\lt 0 \\ (x+1) & \lt 0 \\ x & \lt -1 \end{aligned} $$ so $f(x)$ is concave upward on $(-\infty,-1 )$. Also function $f$ is concave upward on an interval if $f^{\prime\prime}(x) \lt 0$ for all $x$ in the interval, so $$ \begin{aligned} f^{\prime\prime}(x) & =\frac{-4}{(x+1)^{3}} \lt 0 . \end{aligned} $$ when $$ \begin{aligned} (x+1)^{3} & \gt 0 \\ (x+1) & \gt 0 \\ x &\gt -1 \end{aligned} $$ so $f(x)$ is concave downward on $(-1 , \infty)$. Since $f^{\prime\prime}(x)\ne 0 $ for all $x$, $f^{\prime\prime}(x) $ does not exist at $x=-1$ and the sign of $f^{\prime\prime}(x)$ changes from positive to negative, the graph changes from concave upward to concave downward at that point, but $ f(-1))$ does not exist. . So , no inflection point
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