题目内容
(本小题满分12分)
如图,在四棱锥
中,底面
为正方形,侧棱
底面
分别为
的中点.
(1)证明
平面
;
(2)设
,求二面角
的大小.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231506026551597.gif)
如图,在四棱锥
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602156346.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602172301.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602265266.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602343510.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602468321.gif)
(1)证明
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602484264.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602499269.gif)
(2)设
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602546346.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602624425.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231506026551597.gif)
(1)略
(2)二面角
的大小为
.
(2)二面角
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602624425.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602764422.gif)
解法:(1)如图,建立空间直角坐标系
.
设
,则![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602811558.gif)
,
.
取
的中点
,则
.
平面
平面
,
所以
平面
.
(2)不妨设
,则
.
中点![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231506031082100.gif)
又
,
,
所以向量
和
的夹角等于二面角
的平面角.
.
所以二面角
的大小为
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602780390.gif)
设
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602796654.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602811558.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602827908.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602858654.gif)
取
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602874238.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602905607.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602920673.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602936729.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602952490.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602499269.gif)
所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602484264.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602499269.gif)
(2)不妨设
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150603045347.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231506030761464.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150603092232.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231506031082100.gif)
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150603217658.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150603248648.gif)
所以向量
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150603264380.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150603279241.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602624425.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231506033101235.gif)
所以二面角
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602624425.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823150602764422.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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