Question

等比数列的前n项和 等比数列{an}的公比q≠±1,S10=8,则S20/1+q10=?

Answer 1

S20/(1+q^10)

=[a1(1-q^20)/(1-q)]/(1+q^10)

=a1(1-q^20)/[(1-q)(1+q^10)]

=a1(1-q^10)(1+q^10)/[(1-q)(1+q^10)]

=a1(1-q^10)/(1-q)

=S10

=8