已知f(x)为二次函数,不等式f(x)+2<0的解集为(-1, ),且对任意α,β∈R,恒有f(sinα)≤0,f(2+
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已知f(x)为二次函数,不等式f(x)+2<0的解集为(-1,![]() ![]() (1)求函数f(x)的解析式; (2)设b n = ![]() (3)若(2)中数列{b n }的前n项和为S n ,求数列{S n ·cox(b n π)}的前n项和T n 。 |
![已知f(x)为二次函数,不等式f(x)+2<0的解集为(-1, ),且对任意α,β∈R,恒有f(sinα)≤0,f(2+](/uploads/image/z/19133494-70-4.jpg?t=%E5%B7%B2%E7%9F%A5f%EF%BC%88x%EF%BC%89%E4%B8%BA%E4%BA%8C%E6%AC%A1%E5%87%BD%E6%95%B0%EF%BC%8C%E4%B8%8D%E7%AD%89%E5%BC%8Ff%EF%BC%88x%EF%BC%89%2B2%EF%BC%9C0%E7%9A%84%E8%A7%A3%E9%9B%86%E4%B8%BA%EF%BC%88-1%EF%BC%8C+%EF%BC%89%EF%BC%8C%E4%B8%94%E5%AF%B9%E4%BB%BB%E6%84%8F%CE%B1%EF%BC%8C%CE%B2%E2%88%88R%EF%BC%8C%E6%81%92%E6%9C%89f%EF%BC%88sin%CE%B1%EF%BC%89%E2%89%A40%EF%BC%8Cf%EF%BC%882%2B)
(1)依题意
即
令
则
有
得
即
得
∴
;
(2)
则
即
两边取倒数,得
即
∴数列
是首项为
,公差为的等差数列
∴
;
(3)∵
所以
∴
(i)当n为偶数时
(ii)当n为奇数时
综上
。
![](http://img.wesiedu.com/upload/c/81/c81c8e9864a9470818dd4ed211ac0fe3.jpg)
![](http://img.wesiedu.com/upload/2/42/242228a854dbf22e37d1fd7bfd5ba760.jpg)
即
![](http://img.wesiedu.com/upload/d/4d/d4d9495881d7ff203f36c1f5bd8d8717.jpg)
令
![](http://img.wesiedu.com/upload/b/6a/b6a2c383f16f461531cd27d5f6fe3f1c.jpg)
则
![](http://img.wesiedu.com/upload/5/62/562fe2c7bd54d7c3b702b117b1539f02.jpg)
有
![](http://img.wesiedu.com/upload/3/a6/3a66692eeed68193264983f84018281f.jpg)
得
![](http://img.wesiedu.com/upload/7/93/793058ffb5ace446084cdb7f3bf11fc9.jpg)
即
![](http://img.wesiedu.com/upload/1/f5/1f59248932f174ef681d931bb241ceed.jpg)
得
![](http://img.wesiedu.com/upload/c/44/c44d11563af9bcdc4539b8711a3d138a.jpg)
∴
![](http://img.wesiedu.com/upload/2/03/20373c873d6e34e79cd93a4be9212d99.jpg)
(2)
![](http://img.wesiedu.com/upload/f/c9/fc9f1b33afa0adf1d1070ff580ded68f.jpg)
则
![](http://img.wesiedu.com/upload/e/44/e4497bacdbb28938453f5d77cef4bd03.jpg)
即
![](http://img.wesiedu.com/upload/b/80/b80be4d2ecfa68bc54fcebfa698d3dc7.jpg)
两边取倒数,得
![](http://img.wesiedu.com/upload/f/37/f37291529c970c22e337ea5b4efbf338.jpg)
即
![](http://img.wesiedu.com/upload/6/02/6022055c66ef584ee5f5b25ac77c57e9.jpg)
∴数列
![](http://img.wesiedu.com/upload/f/54/f5409dc88fe61cae8ddb8456dbd052c6.jpg)
![](http://img.wesiedu.com/upload/0/19/0199cef7ba6c9910a80d5cb35120afe5.jpg)
∴
![](http://img.wesiedu.com/upload/6/f3/6f3cf3ac402425f757dcd2f4b17d956a.jpg)
(3)∵
![](http://img.wesiedu.com/upload/9/b2/9b2ab518e5875d635a335a5f99c35357.jpg)
所以
![](http://img.wesiedu.com/upload/3/ca/3cacf505cf19e1a2ae84b0b266b5fcf7.jpg)
∴
![](http://img.wesiedu.com/upload/d/48/d483cada50acf5b6d90ffdbd34e3eedc.jpg)
(i)当n为偶数时
![](http://img.wesiedu.com/upload/c/c8/cc82188ccc17ef068847a5603e584227.jpg)
![](http://img.wesiedu.com/upload/9/b1/9b1a84fbc34dd0e3abbb07b49e5290d8.jpg)
(ii)当n为奇数时
![](http://img.wesiedu.com/upload/3/11/311ddb7748a45439d23952e3b7d1ab69.jpg)
![](http://img.wesiedu.com/upload/f/64/f64bac44399bf1a83ef5de39024ed693.jpg)
综上
![](http://img.wesiedu.com/upload/8/e3/8e301dfcb0b13e9ad1c108e5b78fd7de.jpg)
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