yanlucky
幼苗
共回答了15个问题采纳率:93.3% 举报
(1)见解析 (2)
(1)证明:取BD的中点F,连接EF,AF,
则AF=1,EF=
![](https://img.yulucn.com/upload/6/2e/62e7791af8f95117c4025874d069013e_thumb.jpg)
,∠AFE=60°.
由余弦定理知
AE=
![](https://img.yulucn.com/upload/5/9e/59e37feda32411aad1530ba02d16eed3_thumb.jpg)
=
![](https://img.yulucn.com/upload/b/fa/bfae52c34477aa033d54777f32347d2e_thumb.jpg)
.
∵AE
2 +EF
2 =AF
2 ,∴AE⊥EF.
∵AB=AD,F为BD中点.∴BD⊥AF.
又BD=2,DC=1,BC=
![](https://img.yulucn.com/upload/a/a4/aa4999ee731360eaf6dd28e4783902eb_thumb.jpg)
,
∴BD
2 +DC
2 =BC
2 ,
即BD⊥CD.
又E为BC中点,EF∥CD,∴BD⊥EF.
又EF∩AF=F,
∴BD⊥平面AEF.又BD⊥AE,
∵BD∩EF=F,
∴AE⊥平面BDC.
(2)以E为原点建立如图所示的空间直角坐标系,则A
![](https://img.yulucn.com/upload/d/bc/dbc0636175c7b3cc64ab87342067b14b_thumb.jpg)
,
C
![](https://img.yulucn.com/upload/d/81/d811e19c50c0685e75e8bc79148b399b_thumb.jpg)
,
B
![](https://img.yulucn.com/upload/8/9e/89e191478db619bc079c660b28dba18f_thumb.jpg)
,
D
![](https://img.yulucn.com/upload/a/ff/aff2dfe92a443ba70f9a0085f15923ae_thumb.jpg)
,
![](https://img.yulucn.com/upload/3/f2/3f23538dc684f084f882a7a8a8f6cf2c_thumb.jpg)
=(2,0,0),
![](https://img.yulucn.com/upload/7/92/792aaf304078d4e6dc10aa89feb11efa_thumb.jpg)
=
![](https://img.yulucn.com/upload/1/91/191af87da4ee3cd905015445eaa15fd6_thumb.jpg)
,
![](https://img.yulucn.com/upload/8/44/8448b9ebc1fd1456c4836214fbf932d4_thumb.jpg)
=
![](https://img.yulucn.com/upload/4/6f/46f9d60695f44db9bec444f15d0b9ba5_thumb.jpg)
.
设平面ABD的法向量为n=(x,y,z),
由
![](https://img.yulucn.com/upload/1/55/1555de2cc4b9238c282ddbd40207561c_thumb.jpg)
得
取z=
![](https://img.yulucn.com/upload/9/90/9909d6589d694cef8faefcdb7831337d_thumb.jpg)
,
则y=-3,又∵n=(0,-3,
![](https://img.yulucn.com/upload/9/90/9909d6589d694cef8faefcdb7831337d_thumb.jpg)
).
∴cos〈n,
![](https://img.yulucn.com/upload/f/08/f085e79f1d5a537296bd1c663049e0d7_thumb.jpg)
〉=
![](https://img.yulucn.com/upload/d/79/d7906a073b7ad531f1913244a46a1851_thumb.jpg)
=-
![](https://img.yulucn.com/upload/f/e5/fe549ad7a7f2c1c680432535afbd9b9d_thumb.jpg)
.
故直线AC与平面ABD所成角的余弦值为
![](https://img.yulucn.com/upload/8/26/82638e44597f5b5b3617c7c7ff32bc46_thumb.jpg)
.
1年前
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