Question: question using subs

Hi There

I'm getting started with maple and facing a doubt about using subs, somehow it does not seem to work, this is my expression:

> restart;
> theta := omega*t-k*x;
                                omega t - k x
> phi[1] := -(1/2)*H*c*cosh(k*(z+h))*sin(theta)/sinh(k*h);
                   H c cosh(k (z + h)) sin(-omega t + k x)
                   ---------------------------------------
                                 2 sinh(k h)              
> diff(phi[1], t, t);
                                                             2
                H c cosh(k (z + h)) sin(-omega t + k x) omega
              - ----------------------------------------------
                                 2 sinh(k h)                  
> td := simplify(subs(z = 0, %));
                                                          2
                   H c cosh(k h) sin(-omega t + k x) omega
                 - ----------------------------------------
                                 2 sinh(k h)               
> simplify(subs(cosh(k*h)/sinh(k*h) = 1/tanh(k*h), %));
                                                          2
                   H c cosh(k h) sin(-omega t + k x) omega
                 - ----------------------------------------
                                 2 sinh(k h)               
I cannot subs cosh/sinh = tanh. I would like to know why.

Any tips? Thank you