等比数列an中，a1+a2=9, a1*a2*a3=27, Sn为前n项的和，则limSn是多少？

因为{an}是等比数列
所以a1×a3=a2²
a1×a2×a3=a2³=27
所以a2=3
因为a1+a2=9
所以a1=6
所以q=a2/a1=3/6=1/2

于是
lim Sn = a1/(1-q) = 6/(1-(1/2)) = 12

a2=3 a1=6

等比是 q=1/2

Sn=a1*(1-q^n)/(1-q)

=6*1/(1/2)

=12

a1×a3=a2²
a1×a2×a3=a2³=27
a2=3
a1+a2=9
a1=6
q=a2/a1=3/6=1/2

lim Sn = a1/(1-q) = 6/(1-(1/2)) = 12

12

12