题目内容
(本小题共13分)
已知正方形ABCD的边长为1,
.将正方形ABCD沿对角线
折起,使
,得到三棱锥A—BCD,如图所示.
(I)若点M是棱AB的中点,求证:OM∥平面ACD;
(II)求证:
;
(III)求二面角
的余弦值.![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231640244825749.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024513161.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231640245294244.jpg)
已知正方形ABCD的边长为1,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024357408.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024404244.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024420270.gif)
(I)若点M是棱AB的中点,求证:OM∥平面ACD;
(II)求证:
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024451454.gif)
(III)求二面角
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024467327.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231640244825749.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024513161.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231640245294244.jpg)
(1)略
(2)略
(3)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024545268.gif)
解:(I)
在正方形ABCD中,
是对角线
的交点,
O为BD的中点, ---------------------1分
又M为AB的中点,
OM∥AD. ---------------------2分
又AD
平面ACD,OM
平面ACD, ---------------------3分
OM∥平面ACD. ---------------------4分
(II)证明:在
中,![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024560183.gif)
,
, ---------------------5分
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024607128.gif)
,![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024607128.gif)
. ---------------------6分
又
是正方形ABCD的对角线,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024607128.gif)
, --------------------7分
又![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024935407.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024607128.gif)
. --------------------8分
(III)由(II)知
,则OC,OA,OD两两互相垂直,如图,以O为原点,建立
空间直角坐标系
.
则
,
是平面
的一个法向量. --------------------9分
,
,
设平面
的法向量
,则
,
.
即
, --------------------11分
所以
且
令
则
,
,解得
.
--------------------12分
从而
,二面角
的余弦值为
..
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024560183.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024576209.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024591319.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024607128.gif)
又M为AB的中点,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024607128.gif)
又AD
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024654135.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024669194.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024607128.gif)
(II)证明:在
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024716404.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024560183.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024420270.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024779577.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024607128.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024810557.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024607128.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024841331.gif)
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024560183.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024591319.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024607128.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024903334.gif)
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024935407.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024607128.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024451454.gif)
(III)由(II)知
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024451454.gif)
空间直角坐标系
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025153392.gif)
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231640251691622.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025184635.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025215275.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025231674.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025262696.gif)
设平面
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025278270.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025293468.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025356349.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025371342.gif)
即
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231640254031390.gif)
所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025418351.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025449227.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025465234.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025481241.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025496215.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025512362.gif)
--------------------12分
从而
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164025527965.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024467327.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823164024545268.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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