解题方法
1 . 用光线照射物体,在某个平面上得到的影子叫做物体的投影,照射光线叫做投影线,投影所在的平面叫做投影面.由平行光线形成的投影叫做平行投影,由点光源发出的光线形成的投影叫做中心投影.投影线垂直于投影面产生的平行投影叫做正投影,投影线不垂直于投影而产生的平行投影叫做斜投影.物体投影的形状、大小与它相对于投影面的位置和角度有关.如图所示,已知平行四边形
在平面
内的平行投影是四边形
.
![](https://img.xkw.com/dksih/QBM/editorImg/2022/12/15/71cf6fa5-9cb4-4d8d-a907-fa7ccdcd6d3d.png?resizew=314)
![](https://img.xkw.com/dksih/QBM/editorImg/2022/12/15/9fc4f21f-3f78-4976-887c-0642c3365737.png?resizew=314)
![](https://img.xkw.com/dksih/QBM/editorImg/2022/12/15/462c058d-eb41-4d4a-ac0e-89fb806fef7a.png?resizew=314)
(1)若平行四边形
平行于投影面(如图
),求证:四边形
是平行四边形;
(2)在图
中作出平面
与平面
的交线(保留作图痕迹,不需要写出过程);
(3)如图
,已知四边形
和平行四边形
的面积分别为
,平面
与平面
的交线是直线
,且这个平行投影是正投影.设二面角
的平面角为
(
为锐角),猜想并写出角
的余弦值(用
表示),再给出证明.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/411b38a18046fea8e9fab1f9f9b80a5f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e170f206fdbbd834aad7580c727e2cc6.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0d609847e2ff3d64e5a514582c3ead0e.png)
![](https://img.xkw.com/dksih/QBM/editorImg/2022/12/15/71cf6fa5-9cb4-4d8d-a907-fa7ccdcd6d3d.png?resizew=314)
图
![](https://img.xkw.com/dksih/QBM/editorImg/2022/12/15/9fc4f21f-3f78-4976-887c-0642c3365737.png?resizew=314)
图
![](https://img.xkw.com/dksih/QBM/editorImg/2022/12/15/462c058d-eb41-4d4a-ac0e-89fb806fef7a.png?resizew=314)
图
(1)若平行四边形
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/411b38a18046fea8e9fab1f9f9b80a5f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/bdaa19de263700a15fcf213d64a8cd57.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0d609847e2ff3d64e5a514582c3ead0e.png)
(2)在图
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/61128ab996360a038e6e64d82fcba004.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/411b38a18046fea8e9fab1f9f9b80a5f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e170f206fdbbd834aad7580c727e2cc6.png)
(3)如图
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5ca7d1107389675d32b56ec097464c14.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0d609847e2ff3d64e5a514582c3ead0e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/411b38a18046fea8e9fab1f9f9b80a5f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/d3637753af5ce86be9c23a9beb6b5067.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/411b38a18046fea8e9fab1f9f9b80a5f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e170f206fdbbd834aad7580c727e2cc6.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0f85fca60a11e1af2bf50138d0e3fe62.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7021666155884a8aa345ed8eec3d2a01.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c24095e409b025db711f14be783a406c.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c24095e409b025db711f14be783a406c.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c24095e409b025db711f14be783a406c.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/d3637753af5ce86be9c23a9beb6b5067.png)
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