数列{an}的首项为1,数列{bn}为等比数列且bn=,若b10b11=2,则a21= .

数列{a_n}的首项为1,数列{b_n}为等比数列且b_n=,若b₁₀b₁₁=2,则a₂₁=　　　　.

答案

1 024

【解析】因为b₁₀b₁₁=2,所以b₁b₂…b₂₀=(b₁₀b₁₁)¹⁰=2¹⁰.

又b_n=,所以b₁b₂…b₂₀=··…=,

即=2¹⁰,

所以a₂₁=2¹⁰=1 024.

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