I was wondering if anyone could help me. I am really stuck on the following question.

A) How do you show that the derivative of :

f(x) = sech (x)

is:

-sech(x)tanh(x)

B) Find values of C and D so that Csech(Dx) is a solution of:

(d^2u)/(dx^2) - u + u^3 = 0

Any help would be greatly appreciated.