题目内容
(12分)如图所示,在直四棱柱
中,
,
,点
是棱
上一点.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231451590062513.gif)
(Ⅰ)求证:
面
;
(Ⅱ)求证:
;
(Ⅲ)试确定点
的位置,使得平面![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159068420.gif)
平面
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145158756510.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145158756326.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145158772328.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145158787327.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145158803252.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231451590062513.gif)
(Ⅰ)求证:
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159021305.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159021292.gif)
(Ⅱ)求证:
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159037455.gif)
(Ⅲ)试确定点
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145158787327.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159068420.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159084108.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159099339.gif)
(Ⅰ)证明:由直四棱柱,得
,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231451591462810.gif)
所以
是平行四边形,所以
………………………(3分)
而
,
,所以
面
………(4分)
(Ⅱ)证明:因为
, 所以
………(6分)
又因为
,且
,所以
…(7分)
而
,所以
.…………(8分)
(Ⅲ)当点
为棱
的中点时,平面![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159068420.gif)
平面
.………(9分)
取DC的中点N,
,连结
交
于
,连结
.因为N是DC中点,BD=BC,所以
;
又因为DC是面ABCD与面
的交线,而面ABCD⊥面
,所以
……………(11分)
又可证得,
是
的中点,所以BM∥ON且BM=ON,即BMON是平行四边形,所以BN∥OM,所以OM
平面
,所以平面![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159068420.gif)
平面
………………(12分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159115568.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231451591462810.gif)
所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159162336.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159177366.gif)
而
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159193471.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159208504.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159021305.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159021292.gif)
(Ⅱ)证明:因为
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159489540.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159552318.gif)
又因为
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159567304.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159583394.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159598366.gif)
而
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159614514.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159037455.gif)
(Ⅲ)当点
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145158787327.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145158803252.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159068420.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159084108.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159099339.gif)
取DC的中点N,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159786420.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159801262.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159864262.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159879209.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159895372.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159910452.gif)
又因为DC是面ABCD与面
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159942341.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159942341.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145200098616.gif)
又可证得,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159879209.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159801262.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159084108.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145200191316.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159068420.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145159084108.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823145200191316.gif)
略
![](http://thumb2018.1010pic.com/images/loading.gif)
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