设等比数列{an}的前n项和为Sn,若S4=8,S8=24,则S12=
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![设等比数列{an}的前n项和为Sn,若S4=8,S8=24,则S12=](/uploads/image/z/8950074-42-4.jpg?t=%E8%AE%BE%E7%AD%89%E6%AF%94%E6%95%B0%E5%88%97%7Ban%7D%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BASn%2C%E8%8B%A5S4%3D8%2CS8%3D24%2C%E5%88%99S12%3D)
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设等比数列{an}的前n项和为Sn,若S4=8,S8=24,则S12=
设等比数列{an}的前n项和为Sn,若S4=8,S8=24,则S12=
设等比数列{an}的前n项和为Sn,若S4=8,S8=24,则S12=
(s8-s4)/s4=(s12-s8)/(s8-s4)
16/8=(s12-24)/16
s12=56
s4=a1+a1q+a1qq+a1qqq
s8=a1+a1q+a1qq.......+a1qqqqqqq
s8-s4=a1qqqq+a1qqqqq+a1qqqqqq+a1qqqqqqq=16
这样就明白了吧,(s8-s4)/s4=qqqq=2
那么(s12-s8) / s8也等于qqqq等于2
所以s12=2*24+24=72