Calculus 8th Edition

Published by Cengage
ISBN 10: 1285740629
ISBN 13: 978-1-28574-062-1

Chapter 3 - Applications of Differentiation - 3.9 Antiderivatives - 3.9 Exercises - Page 283: 36

Answer

$$ f^{\prime \prime}(x)=8 x^{3}+5 ,\quad f(1)=0 ,\quad f^{\prime}(1)=8 $$ The required function is $$ f(x)=\frac{2}{5} x^{5}+\frac{5}{2}x^{2}+x-\frac{39}{10} $$

Work Step by Step

$$ f^{\prime \prime}(x)=8 x^{3}+5 ,\quad f(1)=0 ,\quad f^{\prime}(1)=8 $$ The general anti-derivative of $ f^{\prime \prime}(x)=8 x^{3}+5 $ is $$ f^{\prime}(x)=2 x^{4}+5x+C $$ To determine C we use the fact that $f^{\prime}(1)=8 $: $$ f^{\prime}(1)=2 (1)^{4}+5(1)+C=8 $$ $ \Rightarrow $ $$ 7+C=8\quad \Rightarrow \quad C=1, $$ so $$ f^{\prime}(x)=2 x^{4}+5x+1 $$ Using the anti-differentiation rules once more, we find that: $$ f(x)=\frac{2}{5} x^{5}+\frac{5}{2}x^{2}+x+D $$ To determine D we use the fact that $f(1)=0 $: $$ f(1)=\frac{2}{5} (1)^{5}+\frac{5}{2}(1)^{2}+(1)+D =0 $$ $ \Rightarrow $ $$ \frac{39}{10}+D=0\quad \Rightarrow \quad D=-\frac{39}{10}, $$ so the required function is $$ f(x)=\frac{2}{5} x^{5}+\frac{5}{2}x^{2}+x-\frac{39}{10} $$
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