Determine whether a formula invalid

[0xGeorgii]

Definition A formula is said to be a tautology (valid) if and only if is true under all its interpretations. A formula is said to be invalid if it is not a tautology.

ref: Symbolic Logic and Mechanical Theorem Proving (Computer Science and Applied Mathematics) 1st Edition,
by Chin-Liang Chang, Richard Char-Tung Lee, p. 11