已知数列{a n}的前n项和S n=n 2-9n，若5＜a k＜8，则a k的值是

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='13507' and id < 1097087 and id<>'1097087' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='13507' and id > 1097087 and id<>'1097087' order by id asc limit 5