Answer
$y'=3\tanh3x$
Work Step by Step
$y=\ln(\cosh3x)$
Start the differentiation process by using the chain rule:
$y'=\dfrac{(\cosh3x)'}{\cosh3x}=...$
Apply the chain rule one more time to evaluate the derivative indicated in the numerator and simplify:
$...=\dfrac{(3x)'\sinh3x}{\cosh3x}=\dfrac{3\sinh3x}{\cosh3x}=3\tanh3x$
