解题方法
1 . 在推导很多三角恒等变换公式时,我们可以利用平面向量的有关知识来研究,在一定程度上可以简化推理过程.如我们就可以利用平面向量来推导两角差的余弦公式:![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5e6276ff5468f5aa9c6eaff479c26cc7.png)
具体过程如下:
如图,在平面直角坐标系
内作单位圆O,以
为始边作角
.它们的终边与单位圆O的交点分别为A,B.
![](https://img.xkw.com/dksih/QBM/editorImg/2022/11/29/3378e1b0-11ac-4e21-89d7-e7bef545c1e9.png?resizew=334)
则![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a98717138350884b83b2bc3335ac3262.png)
由向量数量积的坐标表示,有:
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/437ebce60a1d755209353f0d94462154.png)
设
的夹角为θ,则
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/665d77a90728ca9eb4d63b07dbe89e80.png)
另一方面,由图3.1—3(1)可知,
;由图可知,
![](https://img.xkw.com/dksih/QBM/editorImg/2022/11/29/8e003e58-f755-4f57-ba40-42e3c44c2f0e.png?resizew=348)
.于是
.
所以
,也有
,
所以,对于任意角
有:
(
)
此公式给出了任意角
的正弦、余弦值与其差角
的余弦值之间的关系,称为差角的余弦公式,简记作
.
有了公式
以后,我们只要知道
的值,就可以求得
的值了.
阅读以上材料,利用下图单位圆及相关数据(图中M是AB的中点),采取类似方法(用其他方法解答正确同等给分)解决下列问题:
(1)判断
是否正确?(不需要证明)
(2)证明:![](https://staticzujuan.xkw.com/quesimg/Upload/formula/889623d5e61054f38a35aedd644c9ff5.png)
(3)利用以上结论求函数
的单调区间.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5e6276ff5468f5aa9c6eaff479c26cc7.png)
具体过程如下:
如图,在平面直角坐标系
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7ee31829d0d4d5f779a957d7df8058ab.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c3e5af20b2f8c1fba4470f9650989e51.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/bfa404d3ff313b0a28a76a48d7d87234.png)
![](https://img.xkw.com/dksih/QBM/editorImg/2022/11/29/3378e1b0-11ac-4e21-89d7-e7bef545c1e9.png?resizew=334)
则
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a98717138350884b83b2bc3335ac3262.png)
由向量数量积的坐标表示,有:
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/437ebce60a1d755209353f0d94462154.png)
设
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/538844ce819df320039e394ba92356f8.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/665d77a90728ca9eb4d63b07dbe89e80.png)
另一方面,由图3.1—3(1)可知,
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/655ee7e11f540619722504916419e009.png)
![](https://img.xkw.com/dksih/QBM/editorImg/2022/11/29/8e003e58-f755-4f57-ba40-42e3c44c2f0e.png?resizew=348)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/18eedcc65589e7529da85a578bd0ecb2.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e366809cf946d825277ad151abb374a2.png)
所以
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0a689c643b92f5fafe77fb2c754b0184.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5e6276ff5468f5aa9c6eaff479c26cc7.png)
所以,对于任意角
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c4e288596fa3811dd2c17bded60e82e7.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5e6276ff5468f5aa9c6eaff479c26cc7.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b9e74ca761ffa2566a9851c5ce9ccaaf.png)
此公式给出了任意角
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c4e288596fa3811dd2c17bded60e82e7.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/fd927b4b5a7875528c1b54aa4bb8b2dd.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b9e74ca761ffa2566a9851c5ce9ccaaf.png)
有了公式
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b9e74ca761ffa2566a9851c5ce9ccaaf.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a1455db71a4123b3317dcfce3e2005e0.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/22d521f8d021b20757d7a68107fcef1d.png)
阅读以上材料,利用下图单位圆及相关数据(图中M是AB的中点),采取类似方法(用其他方法解答正确同等给分)解决下列问题:
(1)判断
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/90f93aa4ff886e380c9b7c05dbafd08d.png)
(2)证明:
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/889623d5e61054f38a35aedd644c9ff5.png)
(3)利用以上结论求函数
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/1414c4eb3a476aac49f6a35d62b1f7ac.png)
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2 . 已知函数
,设其最小值为![](https://staticzujuan.xkw.com/quesimg/Upload/formula/01b3ae7e5228fd1acb0d46f6941143a7.png)
(1)求
;
(2)若
,求a以及此时
的最大值.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6fdac28c4c4436ae735bbf02271bb29d.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/01b3ae7e5228fd1acb0d46f6941143a7.png)
(1)求
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/01b3ae7e5228fd1acb0d46f6941143a7.png)
(2)若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/982ecaa2793a1b8fc2c4596f3b9a506c.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/09f86f37ec8e15846bd731ab4fcdbacd.png)
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3 . 已知函数
,给定以下命题:
①
为偶函数;②
为周期函数,且最小正周期为
;③若
,则
恒成立.
正确的命题个数为个.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/acbcaf15f2edb089f80a41b3d9a7c3c0.png)
①
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/4fe7d5809da02c15a43a0e9a898b9086.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/4fe7d5809da02c15a43a0e9a898b9086.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7e9fdc1f8ed0ae44b54a9a2a3aca2db4.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6e33862fa39040863f9ffa2975dedde2.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/018857ec6e498113b3b12a730d9313da.png)
正确的命题个数为个.
A.0 | B.1 | C.2 | D.3 |
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4 .
内角A、B、C的对边分别是a、b、c,已知:
.
(1)求
;
(2)若
边上的中线BD长为
,求
面积;
(3)
,求
内切圆半径的取值范围.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/15c0dbe3c080c4c4636c64803e5c1f76.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0706f95070c2f8b197baebd3462507fb.png)
(1)求
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5963abe8f421bd99a2aaa94831a951e9.png)
(2)若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/1e945ce61be2a8e16d8022a79ec4b8ef.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/50690dab38f4512eb72e18b7f86cf6f6.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/15c0dbe3c080c4c4636c64803e5c1f76.png)
(3)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f8120119749d4bc28067e73fca7d46cd.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/15c0dbe3c080c4c4636c64803e5c1f76.png)
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5 . 已知下列命题:
①函数
在
上单调递减,在
上单调递增;
②若函数
在
上有两个零点,则
的取值范围是
;
③当
时,函数
的最大值为0;
④函数
在
上单调递减;
上述命题正确的是_________ (填序号).
①函数
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/15b0378fde8e23ed9bb6ecacd53eba85.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/60cce332317884d04b38b1ebe8a50d18.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/8938db94f49dcbe0c383fba0241bb0da.png)
②若函数
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7d09ba0bf149692e05b82c819898f7bc.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/4aa0df7f1e45f9de29e802c7f19a4f64.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0a6936d370d6a238a608ca56f87198de.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ab1242ec96ac54e2fd418988d5190a88.png)
③当
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7ccaa6e503b61e9ae78d8439cba2e328.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b4b2c4aeb4675c5e6c3b366229e53f88.png)
④函数
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/587882ac081850caa4447c44a7dbb845.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f6b812beee4aa8957af1c6e1ff98f0f2.png)
上述命题正确的是
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6 . 在
中,内角
的对边分别为
,且
,若
为锐角,则
的最大值为_____ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/15c0dbe3c080c4c4636c64803e5c1f76.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7c0ae8e92dd3119d41f2c830ea526516.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0ace74bfb716753490ebe0e740ff5baa.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b57b8336feafe3b6eb86bb03d0a66d4e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c5db41a1f31d6baee7c69990811edb9f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c4f70bd9a1e18078261ffd8888d8d2a3.png)
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解题方法
7 . 如图,已知正方体
的棱长为1,E为棱
的中点,F为棱
上的点,且满足
,点F、B、E、G、H为面MBN过三点B、E、F的截面与正方体
在棱上的交点,则下列说法错误的是
![](https://img.xkw.com/dksih/QBM/2018/10/18/2056268235685888/2057566685880320/STEM/c53266d5-7634-4d47-8716-dae5b6543d4b.png?resizew=345)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6e09725691ee7851f54c0dee86b2bf55.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/9d88bf46ad08f9677c37eed1d0369329.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2777840758e70e7dbbc18cef8f3d6d2b.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/40863968c33b12a827c8948be66c0f93.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6e09725691ee7851f54c0dee86b2bf55.png)
![](https://img.xkw.com/dksih/QBM/2018/10/18/2056268235685888/2057566685880320/STEM/c53266d5-7634-4d47-8716-dae5b6543d4b.png?resizew=345)
A.HF//BE | B.![]() |
C.∠MBN的余弦值为![]() | D.△MBN的面积是![]() |
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解题方法
8 . 关于函数
,有以下四个结论,其中正确的有( )
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/10f778645f712396f8eff23f2c3801b9.png)
A.![]() ![]() |
B.![]() ![]() |
C.方程![]() |
D.若函数![]() ![]() ![]() |
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9 . 若
,
,
,满足:
,
,则
的值为________ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/d93ff828199006191b58f7cfe430c439.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2e45d54b121fdb6ece00c167f79affe2.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2f71e1a5c60a26f1585d9b81131cfcbb.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/8adcdfb8cc2e8162350f86b29a993f20.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/41b4f4d470d42a016589b07f385f65f4.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/94ca41c520e1ae90775f50af5db1a97c.png)
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10 . 设a,b为正整数,且
是函数
的一个零点,则![](https://staticzujuan.xkw.com/quesimg/Upload/formula/92682840e2a230de346562b2032f8adb.png)
______ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/1d0ac5e4a6ef4f217b2ffb08aea29489.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/579a761c1d6caec68e40f196a9ce633e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/92682840e2a230de346562b2032f8adb.png)
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