1£®ÒÑÖª{a_n}ÊǵȲîÊýÁУ¬a₃£«a₁₁£½40£¬Ôòa₆£­a₇£«a₈µÈÓÚ(¡¡¡¡)

A£®20¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡                     B£®48

C£®60                                                      D£®72

½âÎö£º¡¡¡ßa₆£«a₈£½2a₇£¬

ÓÖa₃£«a₁₁£½2a₇£½40.∴a₇£½20.

∴a₆£­a₇£«a₈£½2a₇£­a₇£½a₇£½20£¬¹ÊÑ¡A.

´ð°¸£º¡¡A

2£®Èô{a_n}ÊǵȲîÊýÁУ¬ÔòÏÂÁÐÊýÁÐÖÐÈÔΪµÈ²îÊýÁеĸöÊýÓÐ(¡¡¡¡)

¢Ù{a_n£«3}¡¡¢Ú{a_n²}¡¡¢Û{a_n£«1£­a_n}¡¡¢Ü{2a_n}¡¡¢Ý{2a_n£«n}