semantics(truth) vs formal system? my first question is can we just define semantics in logic and not define a formal system ?
why do we need a formal system to prove a proposition when for example we know the proposition is true ?
e.g. ( A ^ (A->B) ) -> B    if A is true and A->B is true then B is true

and this can also be shown by truth table. so why we use a formal system to show that the argument is valid when we can just use the semantics to show that . (by construction of truth tables)
since the formulas and the laws of semantics are well defined it is not possible to get to any inconsistencies  (in the sense of semantics !)
i think the counterpart to my question is answered in Formal System and Formal Logical System 
but what about the other way around ?
my secound question is: ShyPerson when answering the above question mentioned that we need some meaning for our propositional formulas thus we define the semantics. can not we just use the same meaning of the connectives in the human language to describe our propositions .
meaning we describe connectives just like the human language ?  

In propositional logic we can "show" validity by truth table.
Truth table supply an algorithm to compute the truth value of every propositional formula; 

in particular, truth tables can be used to tell whether a propositional expression is true for all legitimate input values, that is, logically valid [also called : a tautology].

For first-order logic, we have not a similar algorithm.
F-o logic is undecidable :

unlike propositional logic, first-order logic is undecidable (although semidecidable), provided that the language has at least one predicate of arity at least $2$ (other than equality). This means that there is no decision procedure that determines whether arbitrary formulas are logically valid.  

This is the main reason why we need a deductive calculus for f-o logic : 


because to show that the argument is valid when we cannot just use the semantics to show that . 


