Mathematica help - simplification

[acpn2]

Hello,

I'm trying to find out how I can let Mathematica know what is a variable and what is a parameter when asking it to simplify an expression.

For example: if I have the following expression

\[Beta] \[CapitalOmega] ((1 - \[Gamma] y) \[Xi] \[Sigma] + (x - \[Xi]) \[CapitalOmega]) + a ((-1 + \[Gamma])^2 (y (-1 + \[Beta]) + x \[Delta]))

how to define x and y as my variables so that when I run FullSimplify I get an expression like

constant+coeff*x+coeff*y

?

Otherwise, Mathematica does a simplification that gives me:

a (-1 + \[Gamma])^2 (y (-1 + \[Beta]) +
x \[Delta]) + \[Beta] \[CapitalOmega] (x \[CapitalOmega] - \[Xi] ((-1 + y \[Gamma]) \[Sigma] + \[CapitalOmega]))

Many thanks,
Ana

 

[acpn2]

I think I've sorted it out using Collect (had never used this function before).

Ana