I am having some problems with Simplify or FullSimplify.
Suppose I have some function
f[x_, t_]:= x * g[d] + t* h[d] + ...
where d[x_,t_]:=... is an intermediate variable also expressed in x and t.
Supposed I want to find out D[f[x,t],t]. Now if a human being is to do this, he or she will try to express the results in a mix of d, x, and t in such a way that the end result "looks simple."
If Mathematica were to do it, it will express the end result only in x and t, and this makes the end result extremely complicated.
I tried D[f[x,t],t], Simplify[D[f[x,t],t]], and FullSimplify[D[f[x,t],t]], and none of them will express the end result with intermediate variables to make it simple.
Does anyone know how to do this with Mathematica?