Final answer:
To prove that an expression is a tautology in discrete math, you can use basic rules of logic and truth tables. The disjunctive syllogism can be applied to show that the conclusion is always true.
Step-by-step explanation:
In discrete math, proving an expression is a tautology involves showing that the expression is always true, regardless of the truth values assigned to its variables. This can be done using basic rules of logic and truth tables. A tautology is a statement that is true for all possible truth assignments.
To prove that an expression is a tautology, you can use disjunctive syllogism as a common argument form. This form states that if X is true or Y is true, then the conclusion must be true as well.
By replacing the statements in the expression with variables, you can analyze the argument form and check if the conclusion is always true, irrespective of the truth values of the variables.