问答题
(x)(P(x)Q(x))
问答题(∀x)(P(x)↔Q(x))
(x)P(x)∨(y)Q(y)
问答题(∀x)P(x)∨(∃y)Q(y)
(x)P(x)→(y)Q(y)
问答题(∀x)P(x)→(∃y)Q(y)