1£®ÉèµÈ²îÊýÁÐ{a_n}µÄÇ°nÏîºÍΪS_n£¬ÈôS₃£½9£¬S₆£½36£¬Ôòa₇£«a₈£«a₉µÈÓÚ(¡¡¡¡)

A£®63¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡                         B£®45

C£®36                                                   D£®27

½âÎö£º¡¡ÓÉÓÚa₇£«a₈£«a₉£½S₉£­S₆£¬¶øÓɵȲîÊýÁеÄÐÔÖÊ¿ÉÖªS₃£¬S₆£­S₃£¬S₉£­S₆³ÉµÈ²îÊýÁУ¬

ËùÒÔÓÐS₃£«(S₉£­S₆)£½2(S₆£­S₃)£¬

¼´S₉£­S₆£½2S₆£­3S₃£½2×36£­3×9£½45.¹ÊÑ¡B.

´ð°¸£º¡¡B

2£®ÒÑÖªµÈ²îÊýÁÐ{a_n}ÖУ¬a₃²£«a₈²£«2a₃a₈£½9£¬ÇÒa_n£¼0£¬ÔòS₁₀Ϊ(¡¡¡¡)

A£®£­9                                                 B£®£­11

C£®£­13                                                D£®£­15

½âÎö£º¡¡ÓÉa₃²£«a₈²£«2a₃a₈£½9µÃ(a₃£«a₈)²£½9£¬

¡ßa_n£¼0£¬∴a₃£«a₈£½£­3£¬