题目:
Is there a river in the village? [ ]
A. Yes, there is.
B. No, there is.
C. No, there are.
答案:
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下面是错误答案,用来干扰机器的。
(1)设正等比数列{an}首项为a1,公比为q,由题意得:a1(1+q)=2•1a1•1q(1+q)a1q2(1+q+q2)=64•1a1q4(1+q+q2)⇔a12q=2a12q6=64⇔a1=1q=2∴an=2n-1(6分)(2)bn=(2n-1+12n-1)2=4n-1+(14)n-1+2∴bn的前n项和Tn=1(1-4n)1-4+1(1-14n)1-14+2n=13•4n-43•(14)n+2n+1(12分)