题目内容
(本小题满分10分)如图,在棱长为ɑ的正方体ABCD-A1B1C1D1中,E、F、G分别是CB、CD、CC1的中点.
(1)求证:平面A B1D1∥平面EFG;
(2)求证:平面AA1C⊥面EFG .![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231554377392474.gif)
(1)求证:平面A B1D1∥平面EFG;
(2)求证:平面AA1C⊥面EFG .
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231554377392474.gif)
略
解:(1)在正方体ABCD-A1B1C1D1中连接BD,
∥
,
=
,![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437770263.gif)
为平行四边形 ∴
∥![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437941242.gif)
∵E,F分别为BC,CD的中点
∴EF∥BD ∴EF∥![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437895275.gif)
∵EF
平面GEF,![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437895275.gif)
平面GEF
∴
∥平面GEF
同理
∥平面GEF
∵![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437895275.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438316134.gif)
=![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438503220.gif)
∴平面A B1D1∥平面EFG ……………5分
(2)在正方体ABCD-A1B1C1D1 ∴
平面ABCD
∵EF
平面ABCD ∴
EF
∵ABCD为正方形 ∴AC
BD
∵EF∥BD ∴AC
EF 又∵![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438706383.gif)
∴EF
平面AA1C
∵EF
平面EFG ∴平面AA1C⊥面EFG …………….5分
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437770263.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437801255.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437770263.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437801255.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437770263.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437879258.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437895275.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437941242.gif)
∵E,F分别为BC,CD的中点
∴EF∥BD ∴EF∥
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437895275.gif)
∵EF
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437988135.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437895275.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438253194.gif)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437895275.gif)
同理
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438285254.gif)
∵
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437895275.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438316134.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438285254.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438503220.gif)
∴平面A B1D1∥平面EFG ……………5分
(2)在正方体ABCD-A1B1C1D1 ∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438519270.gif)
∵EF
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437988135.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438519270.gif)
∵ABCD为正方形 ∴AC
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438597108.gif)
∵EF∥BD ∴AC
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438597108.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438706383.gif)
∴EF
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155438597108.gif)
∵EF
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823155437988135.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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