z315315
幼苗
共回答了14个问题采纳率:85.7% 举报
(1)
![](https://img.yulucn.com/upload/a/2d/a2dbabbb830c02608dbd3bfdc4a339ba_thumb.jpg)
; (2)见解析;(3)见解析。
试题分析:(1)因为
![](https://img.yulucn.com/upload/2/4a/24aa97f9c9dc2d05880e78d307f21c05_thumb.jpg)
平面ABCD,所以
![](https://img.yulucn.com/upload/1/ea/1eacc03bc8ae4858f80323a80a0977ab_thumb.jpg)
为
![](https://img.yulucn.com/upload/7/72/77271e2b87dc7fd5262529e68f7e8db8_thumb.jpg)
与平面ABCD所成角,
然后解三角形求出此角即可.
(2)证明面面平行根据判定定理只须证明平面平面A B
1 D
1 内两条相交直线
![](https://img.yulucn.com/upload/9/8f/98fbfc3c6a47ad8fe592aa69bb86d548_thumb.jpg)
和
![](https://img.yulucn.com/upload/b/33/b3349df82ad021b30d873121c8c81fd0_thumb.jpg)
分别平行于平面EFG即可.在证明线面平行时又转化为证明线线平行.
(3)易证:BD
![](https://img.yulucn.com/upload/7/e7/7e712c4addae9126ecaf4d91ea3656b7_thumb.jpg)
平面AA
1 C,再证明EF//BD,因而可证出平面AA
1 C⊥面EFG.
(1)∵
![](https://img.yulucn.com/upload/9/48/948485cb904a0ffd5fa135e8dc79b7a4_thumb.jpg)
平面ABCD=C,在正方体ABCD-A
1 B
1 C
1 D
1 ![](https://img.yulucn.com/upload/2/4a/24aa97f9c9dc2d05880e78d307f21c05_thumb.jpg)
平面ABCD
∴AC为
![](https://img.yulucn.com/upload/7/72/77271e2b87dc7fd5262529e68f7e8db8_thumb.jpg)
在平面ABCD的射影
∴
![](https://img.yulucn.com/upload/1/ea/1eacc03bc8ae4858f80323a80a0977ab_thumb.jpg)
为
![](https://img.yulucn.com/upload/7/72/77271e2b87dc7fd5262529e68f7e8db8_thumb.jpg)
与平面ABCD所成角……….2分
正方体的棱长为
∴AC=
![](https://img.yulucn.com/upload/c/15/c15489687d482a246d205eb2033222a0_thumb.jpg)
,
![](https://img.yulucn.com/upload/7/72/77271e2b87dc7fd5262529e68f7e8db8_thumb.jpg)
=
![](https://img.yulucn.com/upload/a/2d/a2dbabbb830c02608dbd3bfdc4a339ba_thumb.jpg)
………..4分
(2)在正方体ABCD-A
1 B
1 C
1 D
1 连接BD,
![](https://img.yulucn.com/upload/2/fe/2fe6a97bd2f79541d9224c99a37e9022_thumb.jpg)
∥
![](https://img.yulucn.com/upload/6/32/632cd93ff79ff48542c7641d0b69bbf1_thumb.jpg)
,
![](https://img.yulucn.com/upload/2/fe/2fe6a97bd2f79541d9224c99a37e9022_thumb.jpg)
=
![](https://img.yulucn.com/upload/e/a7/ea7021cdf38a34a65aa6e46b3cf29d4c_thumb.jpg)
为平行四边形
∴
![](https://img.yulucn.com/upload/b/33/b3349df82ad021b30d873121c8c81fd0_thumb.jpg)
∥
![](https://img.yulucn.com/upload/9/47/9477cd0a423ea097939fd86b3323a4a1_thumb.jpg)
∵E,F分别为BC,CD的中点
∴EF∥BD∴EF∥
![](https://img.yulucn.com/upload/b/33/b3349df82ad021b30d873121c8c81fd0_thumb.jpg)
…………3分
∵EF
![](https://img.yulucn.com/upload/b/a8/ba822330e0c3986093113bdd1f3581d4_thumb.jpg)
平面GEF,
![](https://img.yulucn.com/upload/0/f6/0f6e2f1927f989097935c2a356f58b91_thumb.jpg)
平面GEF
∴
![](https://img.yulucn.com/upload/b/33/b3349df82ad021b30d873121c8c81fd0_thumb.jpg)
∥平面GEF…………7分
同理
![](https://img.yulucn.com/upload/9/8f/98fbfc3c6a47ad8fe592aa69bb86d548_thumb.jpg)
∥平面GEF∵
![](https://img.yulucn.com/upload/9/8f/98fbfc3c6a47ad8fe592aa69bb86d548_thumb.jpg)
=
∴平面A B
1 D
1 ∥平面EFG……………9分
(3)在正方体ABCD-A
1 B
1 C
1 D
1 ∴
![](https://img.yulucn.com/upload/f/36/f36d4cbd154a0065b56a5502c13e012c_thumb.jpg)
平面ABCD
∵EF
![](https://img.yulucn.com/upload/b/a8/ba822330e0c3986093113bdd1f3581d4_thumb.jpg)
平面ABCD
∴
![](https://img.yulucn.com/upload/f/36/f36d4cbd154a0065b56a5502c13e012c_thumb.jpg)
EF…………10分
∵ABCD为正方形
∴AC
![](https://img.yulucn.com/upload/7/e7/7e712c4addae9126ecaf4d91ea3656b7_thumb.jpg)
BD
∵EF∥BD
∴AC
![](https://img.yulucn.com/upload/7/e7/7e712c4addae9126ecaf4d91ea3656b7_thumb.jpg)
EF………..11分
∴EF
![](https://img.yulucn.com/upload/7/e7/7e712c4addae9126ecaf4d91ea3656b7_thumb.jpg)
平面AA
1 C
∵EF
1年前
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