(Original post by Kolya)
To prove validity you either need to construct a truth table or a derivation of the conclusions from the premises.
I first changed the statements into a boolean algebra, then constructed a truth table and that is why I got as you can see there. 1 and 3 are a tautology, 2 is not. So my questions are first: is my change of statements into a boolean algebra correct? And if it is, does that mean only those statements that are a tautology are right, as is in 1 and 3, and the other as in 2, is not correct?