如圖,在長方體
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中,點
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分別在
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上,且
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,
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.
(1)求證:

平面

;
(2)若規(guī)定兩個平面所成的角是這兩個平面所組成的二面角中的銳角(或直角),則在空間有定理:若兩條直線分別垂直于兩個平面,則這兩條直線所成的角與這兩個平面所成角相等,試根據(jù)上述定理,在

時,求平面
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與平面

所成角的大�。�
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證明:(1)因為
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平面

,
所以
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,

平面
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,得
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.
同理可證
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.
因為
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,

,所以
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平面

.
解:(2)過
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作
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的垂線交
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于

,
因為
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,所以
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平面
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.
設(shè)
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與
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所成角為

,則

即為平面

與平面

所成的角.
以
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點為原點,
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所在直線分別為
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軸,
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軸,
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軸建立空間直角坐標(biāo)系
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,
又
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,
由
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,
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,
可得
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,
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.
因為
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與

所成的角為

,
所以

,
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.
由定理知,平面

與平面

所成角的大小為
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.
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
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
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
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如圖,在三棱錐

中,

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
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
分別是

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
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
.

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
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
;
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
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
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
所成角的大��;
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
為何值時,

在平面

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
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已知三棱錐

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
,
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,

,

,

分別是

,

中點.

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
;
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
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
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
。
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
B

C

D

中,底面邊長為2

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
到平面B

EF的距離d。


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若A

,B

,當(dāng)

取最小值時,

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題型:單選題
已知平面
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內(nèi)有一點
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,平面
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的一個法向量為
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,則下列點
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中,在平面
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內(nèi)的是( )
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