题目内容
(本小题满分12分)如图,在底面为直角梯形的四棱锥
中,
,
平面
.PA=4,AD=2,AB=
,BC=6
(Ⅰ)求证:
平面
;
(Ⅱ)求二面角D—PC—A的大小.![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231515407611335.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540605430.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540621627.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540652240.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540667389.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540699257.gif)
(Ⅰ)求证:
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540714257.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540730356.gif)
(Ⅱ)求二面角D—PC—A的大小.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231515407611335.gif)
(Ⅰ)同解析(Ⅱ)二面角
的大小为![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540792566.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540777325.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540792566.gif)
解法一:(Ⅰ)
平面
,
平面
.
.
又
,
.
,
,
,即
.
又
.
平面
.
(Ⅱ)过
作
,垂足为
,连接
.
平面
,
是
在平面
上的射影,由三垂线定理知
,
为二面角
的平面角.
又
,
,
,
又
,
,
.
由
得
.
在
中,
,
.
二面角
的大小为
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231515417591264.gif)
解法二:(Ⅰ)如图,建立坐标系,
则
,
,
,
,
,
,
,
,
,
.
,
,
又
,
平面
.
(Ⅱ)设平面
的法向量为
,
则
,
,
又
,
,
解得![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542181669.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542196708.gif)
平面
的法向量取为
,
,
.
二面角
的大小为
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231515424611447.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540808279.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540823301.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540839266.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540823301.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540870350.gif)
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540886744.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540901718.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540917389.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540933382.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540948499.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540979344.gif)
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540995390.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541011291.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541026265.gif)
(Ⅱ)过
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541042204.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541073429.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541089200.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541104241.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541135284.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541026265.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541245232.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541104241.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541026265.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541291341.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541307311.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540777325.gif)
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541385670.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541416636.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541432658.gif)
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541447447.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541463464.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541479280.gif)
由
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541510663.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541603749.gif)
在
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541619363.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541650800.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541681720.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541697128.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540777325.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540792566.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231515417591264.gif)
解法二:(Ⅰ)如图,建立坐标系,
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541775349.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541791421.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541806522.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541822362.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541837349.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541853423.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541884576.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541900482.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541915391.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541931393.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541947353.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540979344.gif)
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540995390.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541011291.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541026265.gif)
(Ⅱ)设平面
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542071271.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542087466.gif)
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542103465.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542118464.gif)
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542134600.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542149430.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542165779.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542181669.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542196708.gif)
平面
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541026265.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542243698.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542321392.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542337814.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151541697128.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151540777325.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823151542430608.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231515424611447.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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