已知等比数列{an}的公比为q,前n项和为Sn,若S2011,S2010,S2012成等差数列,且S1=1,则an=__
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已知等比数列{an}的公比为q,前n项和为Sn,若S2011,S2010,S2012成等差数列,且S1=1,则an=______.
当等比数列的公比q=1时,
若S2011,S2010,S2012成等差数列,
则2×2010a1=2011a1+2012a1,
解得a1≠S1=1,
故q≠1,
当q≠1时,若S2011,S2010,S2012成等差数列,a1=S1=1,
则2
a1(1−q2010)
1−q=
a1(1−q2011)
1−q+
a1(1−q2012)
1−q,
解得q=-2,
∴an=(-2)n-1,
故答案为:an=(-2)n-1.
若S2011,S2010,S2012成等差数列,
则2×2010a1=2011a1+2012a1,
解得a1≠S1=1,
故q≠1,
当q≠1时,若S2011,S2010,S2012成等差数列,a1=S1=1,
则2
a1(1−q2010)
1−q=
a1(1−q2011)
1−q+
a1(1−q2012)
1−q,
解得q=-2,
∴an=(-2)n-1,
故答案为:an=(-2)n-1.
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