Answer
$$\frac{1}{5}\ln (\cosh 5x)+c.$$
Work Step by Step
We have
$$\int \tanh(5x)dx=\int \frac{\sinh 5x}{\cosh 5x}dx=\frac{1}{5}\int \frac{\sinh 5x}{\cosh 5x}dx\\
=\frac{1}{5}\int \frac{(\cosh5x)'}{\cosh 5x}=\frac{1}{5}\ln (\cosh 5x)+c.$$
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