# Chapter 6 - Inverse Functions - Review - Exercises - Page 505: 43

$$y' =3\tanh 3x$$

#### Work Step by Step

Given $$y=\ln\cosh (3x)$$ Then \begin{align*} y'&=\frac{3\sinh 3x}{\cosh 3x }\\ &=3\tanh 3x \end{align*}

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