Hi, Inkwan,
Your expression contains a syntax error. It is not possible to use the expressions like [Eta], [Eta]^2 etc., since the square brackets are reserved to only be used for arguments of functions. So I did not use you whole example, but a part of it to which I added two f-independent terms:
expr = 8 (-1 +
5 c^2) (3 Cos[
2 (g + f)] + (3 Cos[2 g + f] + Cos[2 g + 3 f]) e) s^2 Eta^5 +
Sin[g] + Cos[c]
Now, try this:
Select[(expr // Expand), FreeQ[#, f] &]
(* Cos[c] + Sin[g] *)
Have fun!
