I have a function f(x) for which I would need to differentiate and then evaluate it to some product x = y*z. Naively, Mathematica does not accept:
Now, I can somewhat force it by using a rule like so:
D[f[x], x] /. x -> y*z
Now, the problem is that the substitution, for example
rule = f[y*z] -> y*z D[f[x], x] /. x -> y*z /. rule
is not performed at all. Here, I would of course expect the answer to be 1. How can I make this work as intended?