四川省、浙江省的行政中心分别是成都和南京。( )
(Ⅰ)求证:AB∥EF;
(Ⅱ)若PA=PD=AD=2,且平面PAD⊥平面ABCD,求平面PAF与平面AEF所成的二面角的正弦值.
In our school library there a number of books on science, and in these years the number of them growing larger and larger.