formula vs well-formed formula in propositional logic -

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can please explain difference between formula in general , well-formed formula? possible determine truth value of ill-formed formula?

the validity of formula, or truth value (or more evaluation), can assessed if formula formed respect given set of syntax rules.

so formula in general, , in context of math or logic, means well-formed formula.

strictly speaking, can if symbols include '+' | '(' | ')' | 'a'-'z', formula, in strict sense, string formed these symbols. example, a((++z formula.

but 1 must @ rules of construction of formula. , if rules are, example

f,e ::= f + e | (f) | 'a'-'z'

then formula not formed.


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