设{an}是等差数列,bn=(1/2)^an.又b1+b2+b3=21/8,b1*b2*b3=1/8 ,求{an}的通项an.稍微有点步骤.让我看的懂.
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设{an}是等差数列,bn=(1/2)^an.又b1+b2+b3=21/8,b1*b2*b3=1/8 ,求{an}的通项an.稍微有点步骤.让我看的懂.
设{an}是等差数列,bn=(1/2)^an.又b1+b2+b3=21/8,b1*b2*b3=1/8 ,求{an}的通项an.
稍微有点步骤.让我看的懂.
设{an}是等差数列,bn=(1/2)^an.又b1+b2+b3=21/8,b1*b2*b3=1/8 ,求{an}的通项an.稍微有点步骤.让我看的懂.
设bn的公比为q,首项为b
所以b+bq+bq^2=21/8 b^3q^3=1/8
所以bq=1/2 解得 b=1/8,q=4
b=2,q=1/4
当b=1/8,q=4,则d=-2,a1=3,an=5-2n
当b=2,q=1/4 则d=2,a1=-1 an=2n-3
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