Thread: 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?

did you manage to get this far ?

if so all you need to know is that