Hello, my name is inkwan.

I'm so glad to find here since I just started to learn how to use Mathematica.

I'm trying to separate terms in an expression to two parts based on dependency on a specific variable. When the expression was short, it was possible for me to pick terms by hand. However, the expression is getting even complicated and impossible for me to pick terms I want to get.

What I want to do is to remove terms explicitly dependent on 'f' in an attached expression. For instance, I'd like to remove terms such as " (constant)*Cos[3f+4g]."

Here is an example which I've met. Could anyone give me some help to solve this problem?

    (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 
+ 1/e 2 (1 + Cos[f] e)^4 (-4 s^2 Sin[2 (g + f)] 
+ e (2 Sin[f] - 6 c^2 Sin[f] + s^2 (Sin[2 g + f] - 5 Sin[2 g + 3 f]))) (-4 Sin[f] + 12 Cos[2 (g + f)] s^2 Sin[f] 
+ 2 Cos[f]^2 e^2 (-1 + 3 Cos[2 (g + f)] s^2) Sin[f] + 3 e (-1 + 3 Cos[2 (g + f)] s^2) Sin[2 f] - 2 Sin[f] [Eta]^2 + 3 s^2 Sin[2 g 
+ f] [Eta]^2 + s^2 Sin[2 g + 3 f] [Eta]^2 + 6 c^2 Sin[f] (2 + 3 Cos[f] e + Cos[f]^2 e^2 + [Eta]^2)) 
+ 8 (-1 + 3 c^2) [Eta]^3 (-1 + 3 Cos[2 (g + f)] s^2 + 3 Cos[f] e (-1 + 3 Cos[2 (g + f)] s^2) + 3 Cos[f]^2 e^2 (-1 + 3 Cos[2 (g + f)] s^2) 
+ Cos[f]^3 e^3 (-1 + 3 Cos[2 (g + f)] s^2) + [Eta]^3 + 3 c^2 (1 + 3 Cos[f] e + 3 Cos[f]^2 e^2 + Cos[f]^3 e^3 - [Eta]^3)) 
+ 1/e 8 (1 + Cos[f] e)^2 (2 e Sin[ f] (-Sin[f] + 3 c^2 Sin[f] + 3 s^2 Sin[2 g + 3 f]) + 1/4 e^2 Sin[ 2 f] (-2 Sin[f] + 6 c^2 Sin[f] 
+ s^2 (-Sin[2 g + f] + 5 Sin[2 g + 3 f])) + (1 - 3 c^2) Cos[ f] [Eta]^2 + s^2 (4 Sin[f] Sin[2 (g + f)] 
- 3 Cos[f] Cos[2 (g + f)] [Eta]^2)) (-1 + 3 Cos[2 (g + f)] s^2 + 3 Cos[f] e (-1 + 3 Cos[2 (g + f)] s^2) 
+ 3 Cos[f]^2 e^2 (-1 + 3 Cos[2 (g + f)] s^2) + Cos[f]^3 e^3 (-1 + 3 Cos[2 (g + f)] s^2) + [Eta]^3 + 3 c^2 (1 + 3 Cos[f] e 
+ 3 Cos[f]^2 e^2 + Cos[f]^3 e^3 - [Eta]^3)) + 1/e (1 + Cos[f] e)^2 (3 Cos[ 2 (g + f)] + (3 Cos[2 g + f] 
+ Cos[2 g + 3 f]) e) s^2 [Eta]^2 (2 Cos[ f] e^2 (-2 (3 + Cos[2 f]) + c^2 (26 + 6 Cos[2 f] - 8 Cos[2 (g + f)]) 
+ (Cos[2 g] + 18 Cos[2 (g + f)] + 5 Cos[2 (g + 2 f)]) s^2) + 8 e (c^2 (5 + 3 Cos[2 f] - 2 Cos[2 (g + f)]) 
- 2 Cos[f] (Cos[f] - 3 Cos[2 g + 3 f] s^2)) + 4 (2 (-1 + 3 c^2) Cos[f] [Eta]^2 + s^2 (-8 Sin[f] Sin[2 (g + f)] 
+ 6 Cos[f] Cos[2 (g + f)] [Eta]^2))) + 1/e 2 (1 + Cos[f] e)^3 s^2 Sin[ 2 (g + f)] [Eta]^2 (-6 e (-4 l + 4 f 
+ 3 s^2 Sin[2 g] + 2 Sin[2 f] - 6 s^2 Sin[2 (g + f)] + c^2 (20 l - 20 f - 6 Sin[2 f] + 4 Sin[2 (g + f)]) - 3 s^2 Sin[2 (g + 2 f)]) 
+ e^2 (-2 (13 Sin[f] + Sin[3 f] + c^2 (-63 Sin[f] - 3 Sin[3 f] + 12 Sin[2 g + f] + 4 Sin[2 g + 3 f])) + 3 s^2 (-Sin[2 g - f] 
+ 11 Sin[2 g + f] + 5 Sin[2 g + 3 f] + Sin[2 g + 5 f])) + 4 (-2 Sin[f] (2 + [Eta]^2) + 6 c^2 Sin[f] (2 + [Eta]^2) 
+ s^2 (12 Cos[2 (g + f)] Sin[ f] + (3 Sin[2 g + f] + Sin[2 g + 3 f]) [Eta]^2))))

Thank you very much and have a wonderful month!!

:: Inkwan