Answer
$$y' =3\tanh 3x$$
Work Step by Step
Given $$ y=\ln\cosh (3x) $$
Then
\begin{align*}
y'&=\frac{3\sinh 3x}{\cosh 3x }\\
&=3\tanh 3x
\end{align*}
Calculus 8th Edition
by Stewart, James
Published by
Cengage
ISBN 10:
1285740629
ISBN 13:
978-1-28574-062-1
Chapter 6 - Inverse Functions - Review - Exercises - Page 505: 43
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