请帮我解答一下2008全国卷2理数的第20题!这题真的好难!手机上不了图,
来源:学生作业帮助网 编辑:作业帮 时间:2024/06/30 01:14:13
![请帮我解答一下2008全国卷2理数的第20题!这题真的好难!手机上不了图,](/uploads/image/z/12103080-24-0.jpg?t=%E8%AF%B7%E5%B8%AE%E6%88%91%E8%A7%A3%E7%AD%94%E4%B8%80%E4%B8%8B2008%E5%85%A8%E5%9B%BD%E5%8D%B72%E7%90%86%E6%95%B0%E7%9A%84%E7%AC%AC20%E9%A2%98%21%E8%BF%99%E9%A2%98%E7%9C%9F%E7%9A%84%E5%A5%BD%E9%9A%BE%21%E6%89%8B%E6%9C%BA%E4%B8%8A%E4%B8%8D%E4%BA%86%E5%9B%BE%2C)
请帮我解答一下2008全国卷2理数的第20题!这题真的好难!手机上不了图,
请帮我解答一下2008全国卷2理数的第20题!这题真的好难!手机上不了图,
请帮我解答一下2008全国卷2理数的第20题!这题真的好难!手机上不了图,
20解 Ⅰ依题意1 13nn n n nS S a S 即12 3nn nS S 由此得113 2( 3 )n nn nS S ·························································································· 4分 因此所求通项公式为 13 ( 3)2n nn nb S a *nN① ·············································································· 6分 Ⅱ由①知13 ( 3)2n nnS a *nN 于是当2n≥时 a S S 1 1 23 ( 3) 2 3 ( 3) 2n n n na a 1 22 3 ( 3)2n na 1 214 3 ( 3)2n nn na a a 2232 12 32nna 当2n≥时 21312 3 02nn na a a ≥ ≥ 9a ≥ 又2 1 13a a a 综上所求的a的取值范围是9