Enderton 2.2.19

An formula is one of the form , where is universal.

(a) Show that if an sentence in a language not containing function symbols (not even constant symbols) is true in , then it is true in some finite substructure of .

(b) Conclude that is not logically equivalent to any sentence.

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(a) We have for some by assumption. We have to show that there are some finite substructure of such that

for the above .

How do I proceed from here?

Thanks.