题目内容
如图,在矩形
中,
是
的中点,以
为折痕将
向上折起,使
为
,且平面
平面
.
(Ⅰ)求证:
;
(Ⅱ)求直线
与平面
所成角的正弦值.![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231639493982145.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949179301.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949211534.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949211242.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949242234.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949257405.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949273210.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949289221.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949304409.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949320407.gif)
(Ⅰ)求证:
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949335441.gif)
(Ⅱ)求直线
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949351234.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949367280.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231639493982145.gif)
(1)略
(2)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949429333.gif)
(Ⅰ)在
中,
,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231639494911753.gif)
在
中,
,
∵
,∴
.…………………………………………..2分
∵平面
平面
,且交线为
,
∴
平面
.
∵
平面
,∴
.………………………………………………6分
(Ⅱ
)(法一)设
与
相交于点
,由(Ⅰ)知
,
∵
,∴
平面
,
∵
平面
,∴平面
平面
,且交线为
,……………………………………7分
如图19-2,作
,垂足为
,则
平面
,连
结
,则
是直
线
与平面
所成的角.…………………………………………..9分
由平面几何的知识可知
,∴
.
在
中,
,
在中,
,可求得
.∴
。
所以直线与平面所成角的正弦值为
。
…………………………………………..14分
(法二)向量法(略)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949445456.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949476670.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231639494911753.gif)
在
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949507464.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949538664.gif)
∵
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949554591.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949569322.gif)
∵平面
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949585404.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949320407.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949242234.gif)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949632260.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949647384.gif)
∵
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949663271.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949647384.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949694448.gif)
(Ⅱ
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/2014082316394971072.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949351234.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949741241.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949772200.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949694448.gif)
∵
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949803456.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949819270.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949850283.gif)
∵
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949663271.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949647384.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949913298.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949850283.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949959257.gif)
如图19-2,作
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949991339.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163950022213.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163950037261.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949367280.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/2014082316395008472.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163950100238.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163950131400.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/2014082316395016272.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949351234.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949367280.gif)
由平面几何的知识可知
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163950209645.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163950240616.gif)
在
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163950287445.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163950303954.gif)
在中,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163950334618.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163950349427.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163950365673.gif)
所以直线与平面所成角的正弦值为
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823163949429333.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/2014082316395041272.gif)
(法二)向量法(略)
![](http://thumb2018.1010pic.com/images/loading.gif)
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