Replace small expressions in larger expressions?

Question

How can I replace small expressions in larger expressions?
Simple example
Replace[a^2 + b^2 + 2*a*b + x, (a + b)^2 -> c]
I want to get c + x but the output is a^2 + b^2 + 2*a*b + x.

Alexei Boulbitch · Answer

Try this.
First:
s = Solve[(a + b)^2 == c, a][[1, 1]]

(*  a -> -b - Sqrt[c]  *)

second:
a^2 + b^2 + 2*a*b + x /. s // Simplify

(*  c + x   *)

Have fun!

Replace small expressions in larger expressions?

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