1 . 将正方体的表面的对角线称为面对角线.若a,b是任意两条面对角线,则a,b所成角的大小为______ (写出所有可能的值)
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2 . 空间中A,B,C,D四点任意两点间距离都等于a,E为
中点,在由A,B,C,D确定的四个等边三角形中,求与
异面的三角形中线与
所成角的大小.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0dc5c9827dfd0be5a9c85962d6ccbfb1.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/68a83fdd2ba72a2dba0b6b10bb3e06b9.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/68a83fdd2ba72a2dba0b6b10bb3e06b9.png)
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3 . 已知椭圆
的左、右焦点分别为
和
,经过点
且斜率为k的直线l交椭圆
于B,C两点,其中点C在第二象限.如图所示,将
的上半部分(半椭圆)沿着长轴翻折使得点C翻折至点A且二面角
为直二面角.设三角形
和三角形
的周长分别为
和
.
![](https://img.xkw.com/dksih/QBM/editorImg/2023/1/18/ba44a91d-f312-406b-b629-1d8e0330619f.png?resizew=335)
(1)证明:
;
(2)若
,求异面直线
和
所成角的大小;
(3)若
,求k的值.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f639240bf9ab7a9a68e779cd51ab2641.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f5076289823db419f94e9c0c8f4aafd9.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a3fb78c5f885034612c0e030b920143d.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f5076289823db419f94e9c0c8f4aafd9.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5b734e8f1546481e3eb4976008a045de.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5b734e8f1546481e3eb4976008a045de.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5465c4f86cbc6cc2c9ba7adbc2060b8b.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/045a830b449a9b95a51688c63d80ba15.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/cef419f0740dae5db5cc51675bc51afb.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a1256c57833e0d2d6b58e7fa20576eed.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a94cde8f1ae6799f3fe64ed059b485fc.png)
![](https://img.xkw.com/dksih/QBM/editorImg/2023/1/18/ba44a91d-f312-406b-b629-1d8e0330619f.png?resizew=335)
(1)证明:
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/800ce9c8dfb28051661e98015f4041c2.png)
(2)若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f247c2d094e3a2b5df9178d5f55fbf39.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e86dbcf83cd5d3421b3eed7be7dab32d.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/acbad65b3d744b70da2480eee1cdb587.png)
(3)若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/33a065ed0c5b57b6be8f3be52249c33f.png)
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