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   *nN① ·············································································· 6分 Ⅱ由①知13 ( 3)2n nnS a  *nN 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  