已知数列{an}是a1=1的等比数列,{bn}是首项为1的等差数列,a5+b3=21,a3+b5=13,①求{an}与.
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已知数列{an}是a1=1的等比数列,{bn}是首项为1的等差数列,a5+b3=21,a3+b5=13,①求{an}与...
已知数列{an}是a1=1的等比数列,{bn}是首项为1的等差数列,a5+b3=21,a3+b5=13,①求{an}与{bn}通项②求{bn/2an}的前n项和sn
已知数列{an}是a1=1的等比数列,{bn}是首项为1的等差数列,a5+b3=21,a3+b5=13,①求{an}与{bn}通项②求{bn/2an}的前n项和sn
(1)A3+B5=21=1+2d+q^4
A5+B3=13=1+4d+q^2
所以An=1+2(n-1)=2n-1
Bn=2^(n-1)
(2)
设Cn=An/Bn=(2n-1)/2^(n-1)
C1=1,C2=3/2,C3=5/4,C4=7/8
Sn=1+3/2+5/4+7/8+……+(2n-3)/2^(n-2)+(2n-1)/2^(n-1)
A5+B3=13=1+4d+q^2
所以An=1+2(n-1)=2n-1
Bn=2^(n-1)
(2)
设Cn=An/Bn=(2n-1)/2^(n-1)
C1=1,C2=3/2,C3=5/4,C4=7/8
Sn=1+3/2+5/4+7/8+……+(2n-3)/2^(n-2)+(2n-1)/2^(n-1)
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