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=

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