Question

Two statements are logically equivalent when:

A. The two statements are true in virtue of their logical structure alone, i.e. the two statement are always true.
B. The first statement implies the second, i.e. if the first statement is true, so is the second.
C. The two statements agree in point of truth or falsehood in virtue of their logical structure alone, i.e. the two statement are true or false in exactly the same conditions.
D. The two statements are false in virtue of their logical structure alone, i.e. the two statement are always false.

Answer 1

C. The two statements agree in point of truth or falsehood in virtue of their logical structure alone, i.e. the two statement are true or false in exactly the same conditions.

Explanation:

For two statements to be logically equivalent, their truth values (true or false) must be the same for every variation of their constituent variables. In other words, if the truth tables of both statements are the same for every possible value of their variables, then they are logically equivalent.

For example;

The two statements P ∩ (Q U R) and (P ∩ Q) ∪ (P ∩ R) are logically equivalent.

If P, Q and R are all true, then;

P ∩ (Q U R) = true

(P ∩ Q) ∪ (P ∩ R) = true

If P, Q and R are all true, then;

P ∩ (Q U R) = false

(P ∩ Q) ∪ (P ∩ R) = false

If P = false, Q = true and R = true, then;

P ∩ (Q U R) = false

(P ∩ Q) ∪ (P ∩ R) = false

Checking for all other possible combinations of truth values of P, Q and R will always give the same results for the two statements, therefore, they are logically equivalent.