Another method, perhaps easier to be made more general is: Find the "variables" - meaning the expressions not being dependet of A or B

vv = Complement[Variables[y], {A, B}]

Now find the coefficients of these variables

cc = Coefficient[y, #] & /@ vv

and then find the factors of these

Collect[y, cc]

Thank you!

This idea worked pretty neat! Precisely what I intended to obtain as an answer form of the equation. Very easy to understand.

Now I will try to apply those methods into a[t]. Because I need to simplify redundant functions for applying linear solving methods (non singular matrix).

I'm grateful! Thanks again.

Thanks for the help! You are right, sorry for the mistake, Ixx[t] is Ixx; e this also aplies to Iyy and Izz.

However, I've tried to use it and still isn't what I want as an answer.

To put in other example,...

Let's say that I have this expression:

y = A*Cos [\[Theta]] + (B/A)*Sin[\[Theta]] + (B*A)*x + 
  A*Tan[x] + (B/A)*x^2

And, what I want to know is how to rewrite it like this, using Mathematica methods:

y = A*(Cos [\[Theta]]+Tan[x]) + (B/A)*(Sin[\[Theta]]+x^2) + (B*A)*x

Because I want "y" as a sum of the constant terms {A,B} in any combination between them, but also multiplying a function that do not include them, like above.

For example, this format o y, applying Apart for "B"

Apart[ y , {B}]

Results in:

{(B (A^2 x + x^2 + Sin[\[Theta]]))/A + A (Cos[\[Theta]] + Tan[x])}

Which doesn't help, because the first polynomial term contains "A", that I want only as a coefficient.

Thanks again for the help.

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