Simplifying with assumed pattern

Question

I have following polynomial

1/16 + (3 Cos[β]^2)/16 - (3 Cos[γ]^2)/16 + 
 3/16 Cos[β]^2 Cos[γ]^2 - (3 Cos[χ])/16 - 
 9/16 Cos[β]^2 Cos[χ] - 3/16 Cos[γ]^2 Cos[χ] + 
 3/16 Cos[β]^2 Cos[γ]^2 Cos[χ] - (
 3 Sin[β]^2)/16 - 3/16 Cos[γ]^2 Sin[β]^2 + 
 9/16 Cos[χ] Sin[β]^2 - 
 3/16 Cos[γ]^2 Cos[χ] Sin[β]^2 + (
 3 Sin[γ]^2)/16 - 3/16 Cos[β]^2 Sin[γ]^2 + 
 3/16 Cos[χ] Sin[γ]^2 - 
 3/16 Cos[β]^2 Cos[χ] Sin[γ]^2 + 
 3/16 Sin[β]^2 Sin[γ]^2 + 
 3/16 Cos[χ] Sin[β]^2 Sin[γ]^2

and I want to simplify it with pattern matching i.e. each time (during simplify) following pattern

1/2 (-1 + 3 Cos[β]^2)

is found, it is onward substituted with expression "A".

Answer 1

Solve for Cos[β] to get it in terms of A, and then substitute Cos[β]

cosB = Solve[A == (1/2 (-1 + 3 Cos[β]^2)), Cos[β]][[1]];
expr /. {Sin[β] -> Sqrt[1 - Cos[β]^2]} /. cosB // Simplify
(*1/4 (A + 2 (-1 + A) Cos[2 γ] Cos[χ/2]^2 - 3 A Cos[χ])*)

Answer 2

poly = 1/16 + (3 Cos[β]^2)/16 - (3 Cos[γ]^2)/16 + 
   3/16 Cos[β]^2 Cos[γ]^2 - (3 Cos[χ])/16 - 
   9/16 Cos[β]^2 Cos[χ] - 
   3/16 Cos[γ]^2 Cos[χ] + 
   3/16 Cos[β]^2 Cos[γ]^2 Cos[χ] - (3 \
Sin[β]^2)/16 - 3/16 Cos[γ]^2 Sin[β]^2 + 
   9/16 Cos[χ] Sin[β]^2 - 
   3/16 Cos[γ]^2 Cos[χ] Sin[β]^2 + (3 \
Sin[γ]^2)/16 - 3/16 Cos[β]^2 Sin[γ]^2 + 
   3/16 Cos[χ] Sin[γ]^2 - 
   3/16 Cos[β]^2 Cos[χ] Sin[γ]^2 + 
   3/16 Sin[β]^2 Sin[γ]^2 + 
   3/16 Cos[χ] Sin[β]^2 Sin[γ]^2;

Using straightforward simplification

poly2 = poly // FullSimplify

(*  1/32 (2 + 3 Cos[2 (β - γ)] + 
   3 Cos[2 (β + γ)] + 
   Cos[2 β] (6 - 18 Cos[χ]) - 6 Cos[χ] - 
   6 Cos[2 γ] (1 + 2 Cos[χ] Sin[β]^2))  *)

Using variable substitution

poly3 = poly /. 
   Assuming[{-1/2 < A <= 1, C[1] == 0}, 
    Solve[A == 1/2 (-1 + 3 Cos[β]^2), β][[1]] // 
     Simplify] // FullSimplify

(*  1/4 (A - 3 A Cos[χ] + (-1 + A) Cos[2 γ] (1 + Cos[χ]))  *)

Verifying,

poly == poly2 == (poly3 /. 
    A -> 1/2 (-1 + 3 Cos[β]^2)) // Simplify

(*  True  *)