Answer
$$\int \tanh x dx
=\ln |\cosh x| +c.$$
Work Step by Step
Let $ u=\cosh x $, then $ du=\sinh x dx $ and hence
$$\int \tanh x dx= \int \frac{\sinh x}{\cosh x} dx =\int \frac{du}{u} \\
= \ln |u|+c
=\ln |\cosh x| +c.$$
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