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I am deriving an expression and the output of it is a very long mathematical expression. I want to minimize the equation by substituting some other variables. For example:

enter image description here This is just an example equation, is it possible for mathematica to automatically do this?

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    $\begingroup$ Have a look at Replace and ReplaceAll. An example using the latter would be output /. {Sin[x + y] -> a, Cos[x + y] -> b}. $\endgroup$
    – Natas
    Nov 12 '20 at 17:54
  • $\begingroup$ Is it possible for Mathematica to provide this A and B in the solution? $\endgroup$ Nov 12 '20 at 18:13
  • $\begingroup$ This question has already been asked and aswered here. $\endgroup$
    – yarchik
    Nov 12 '20 at 18:26