设{an}是公比为q的等比数列,Sn是它的前n项和.若{Sn}是等差数列,则q=______.
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设{an}是公比为q的等比数列,Sn是它的前n项和.若{Sn}是等差数列,则q=______.
设首项为a1,则
s1=a1,
s2=a1+a1q
s3=a1+a1q+a1q2
由于{Sn}是等差数列,
故2(a1+a1q)=a1+a1+a1q+a1q2
q2-q=0
解得q=1.
故答案为:1.
s1=a1,
s2=a1+a1q
s3=a1+a1q+a1q2
由于{Sn}是等差数列,
故2(a1+a1q)=a1+a1+a1q+a1q2
q2-q=0
解得q=1.
故答案为:1.
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