"There is ______ in today's newspaper, so you needn't read it," the father says to his son.
关节既牢固又灵活，与此功能特点无关的是
如图，在矩形ABCD中，对角线AC、BD相交于点O，点E、F分别是AO、AD的中点，若AB=6，∠BDA=30°，求EF的长度．
