## Calculus: Early Transcendentals 8th Edition

$y'=3\tanh3x$
$y=\ln(\cosh3x)$ 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: $...=\dfrac{(3x)'\sinh3x}{\cosh3x}=\dfrac{3\sinh3x}{\cosh3x}=3\tanh3x$