Hi dear All Sorry for a stupid question, but I am really newbe in M. I have some function, for example D[y[r],r]. Now I want to substitute y[r] to Sin[r] and to ask Mathematicato make all the calculations. (i.e I want to get Cos[x]) If I make smth like D[y[r],r]/.y[r]->Sin[r] or D[y[r], r] /. y -> Sin[r] it doesnt work. How do to it? Thank you very much (In the real life I have a set of Einstein equations, that I solve for some arbitrary function of r, θ,χ and then try to subs exact form).