(1)∵SA⊥平面EFGH,GH⊆平面EFGH,∴SA⊥GH
又∵在平面SAB内,SA⊥AB,∴AB∥GH
∵AB⊈平面EFGH,GH⊆平面EFGH,∴AB∥平面EFGH;…(6分)
(2)∵AB∥平面EFGH,AB⊆平面ABC,平面ABC∩平面EFGH=EF
∴AB∥EF
又∵AB∥GH,∴EF∥GH…(10分)
(3)∵SA⊥平面EFGH,SA⊆平面SAC
∴平面SAC⊥平面EFGH,交线为FG
∵EF∥GH,EF⊥FG,∴GH⊥FG
∵GH⊆平面EFGH,
∴GH⊥平面SAC.…(140分)