First order logic variable without quantifiers

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In First Order Logic is it possible to define a formula as :

p(X,a)

where X is a variable and a a constant ? Or i must use quantifiers if i use a variable in a predicate ?

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It's a formula, yes. It's just not sentence.

A sentence is a formula without any free variables. But syntactically, we build up sentences using formulas. For example, a sentence like $\forall x \ p(x,a)$ starts with the formula $p(x,a)$, and then adds a quantifier.