You use Symbolic Toolbox for that, right?

Give us an example and also the error you obtain.

I am using a base relation in which contains 5 parameters and each of them is a seperate formula that has also differential function,the error solved but after substitution of them ,differential function(diff(u,r)) that existed in the base relation and i wanted replace u of it by a formula, had been vanished.I mean it did not replace any formula into the diff function and derivate formula in according to r

syms x t  % Create symbolic Vars

f(x,t) = sin(x) + t*cos(x) % Create symbolic function

diff(f,x) % Derive f in x

diff(f,t) % Derive f in t