1。直三棱柱ABC-A1B1C1中,∠ACB=90°,EFG分别是AA1,AC,BB1的中点,且CG⊥C1G,(1)CG‖平面BEF,(2)CG⊥平面A1C1G
2。在棱长为2的正方体ABCD-A1B1C1D1中,O为BD1的中点,M为BC的中点,N为AB的中点,P为BB1的中点,(1)BD1垂直B1C(2)BD1垂直平面MNP(3)异面直线B1O与C1M所成角的余弦值。
3。四棱锥P-ABCD的底面是正方形,PD垂直底面,PD=AD=a(1)二面角A=PB-D的大小(2)在线段PB上是否存在一点E,是的PC⊥ADE,存在确定位置,不存在说明理由。
真的很急,在线等,求高手帮帮忙,谢了!
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