问答题
(∀x)(A(x)↔B(x))=>(∀x)A(x)↔(∀x)B(x)
问答题 (x)(A(x)→B(x))=>(x)A(x)→(x)B(x)
问答题 (x)(B→A(x))B→(x)A(x)
问答题 (x)(A(x)→B)(x)A(x)→B