令P，Q和R为三个原子介词断言。令X表示(P v Q)→R，Y表示(P→R)v(Q→R)。以下是重言式中的哪一项?
(A) X≡Y
(B) X→Y
(C) Y→X
(D) ¬Y→X答案： (B)
解释：
Answer : [ B ]
X = (P ⋁ Q) → R
= ~(P ⋁ Q) ⋁ R
= (~P ⋀ ~Q) ⋁ R
= (~P ⋁ R) ⋀ (~Q ⋁ R)
= (P → R) ⋀ (Q → R)
X → Y is true as (A ⋀ B) → (A ⋁ B) is always TRUE
but reverse implication is not always true. 这个问题的测验