①若m∥α,n⊥m,则n⊥α  ②若m⊥α,n⊥m,则n∥α  ③若α⊥β,γ⊥β,则α∥γ  ④若m⊥α,mβ,则α⊥β

A.1                    B.2                C.3              D.4

A.m⊥α,nβ,m⊥nα⊥β                    B.α∥β,m⊥α,n∥βm⊥n

C.α⊥β,m⊥α,n∥βm⊥n                     D.α⊥β,α∩β=m,n⊥mn⊥β

                (1)

               (2)

(1)证明AC⊥BO₁;

(2)求二面角O-AC-O₁的大小.

(1)求证:平面PAB⊥平面ABC;

(2)求二面角P-AC-B的大小.

求证:β⊥γ.