Question

已知公比为1/2的等比数列 若a1+a4+a7+```+a97=100 则a3+a6+a9+```+a99=

Answer 1

a3=a1*(1/2)^2

.

a99=a97*（1/2）^2

a3+a6+a9+```+a99=(a1+a4+a7+```+a97)*(1/2)^2=25