i have trouble with understand this type of questions , i know how i can determine if the sentences is valid or unsatisfiable in Propositional logic , but in FOL i can't for example , i have the following sentences in FOL : 1 = 2 the professor said this is neither valid nor unsatisfiable but , i think it is always unsatisfiable , because 1 != 2 !!! can someone explain to me how can i answer this type of question thank you all 
how can i know if a First order logic sentences is valid or unsatisfiable or neither?
artificial intelligence;first order logic