已知等比数列(an)中,a1+a2=9,a1a2a3=27,sn=?

问题描述:

已知等比数列(an)中,a1+a2=9,a1a2a3=27,sn=?
详细
谢谢,

a1a2a3=a2^3=27
a2=3
a1+a2=9
a1=6
q=a2/a1=3/6=1/2
Sn=a1*(1-q^n)/(1-q)=12[1-(1/2)^n]