A:若m⊂β,α⊥β,则m⊥α
B:若α∩β=m,β∩γ=n,m∥n ,则α∥β
C:若α⊥γ,α⊥β,则β∥γ
D:若m⊥β,m∥α,则α⊥β
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如图,从一艘船上测得一个灯塔的方向是北偏西47°,那么这艘船在这个灯塔的( )
如图,已知矩形ABCD沿着直线BD折叠,使点C落在C′处,BC′交AD于E,AD=8,AB=4,则DE的长为