1. ## differentiating cosh

f(x)=ln(cosh(7x+9))
I have to differentiate the function above. I think you should use the chain rule but it doesnt seem to be working for me. The answer is 7tanh(7x+9). But i dont know where the the tanh comes from. Can anybody help?

2. $\displaystyle f(x) = \ln( \cosh(7x+9))$

$\displaystyle f'(x) = \frac{7 \sinh(7x+9)}{\cosh(7x+9)}$

did you manage to get this far ?

if so all you need to know is that $\displaystyle \tanh x = \frac{\sinh x}{\cosh x}$