Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 3 - Differentiation - 3.5 Higher Derivatives - Exercises - Page 136: 38

Answer

$$ y=108x-216.$$

Work Step by Step

Since $ f(x)=x^4$, then $ y=f'(x)=4x^3$. To find the tangent line of $ y $, we have $ y'=12x^2 \Longrightarrow m=y'(3)=108.$ The tangent at $ x=3$ is given by $$ y=108x+c.$$ To find $ c $, we have $ y(3)= 4(27)=108$ and hence $108=108(3)+c $, then $ c=-216$. Then the line is $$ y=108x-216.$$
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