A:might borrow
B:must have borrowed
C:could have borrowed
D:ought to borrow
享有“近代史的百科全书”赞誉的报纸是( )
(Ⅰ)求证:AB⊥平面SAC
(Ⅱ)若SA=2AB=3AC,求二面角S﹣BD﹣A的余弦值.
下面对有关课文内容理解不正确的一项是( )