记公差d≠0的等差数列{an}的前n项和为Sn,一直a1=2+根号2,S3=12+3根...
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发布时间：2024-10-23 20:47
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时间：2024-11-06 11:25
s(n) = na + n(n-1)d/2 = n[2+2^(1/2)] + n(n-1)d/2,
12 + 3(2)^(1/2) = s(3) = 3[2+2^(1/2)] + 3d, d = 2.
a(n) = 2+2^(1/2) + 2(n-1).
b(n) = a(n) - 2^(1/2) = 2+2(n-1)=2n.
b[n(k)] = 2n(k) = b[n(1)]*q^(k-1) = 2n(1)*q^(k-1) = 2q^(k-1),
b[n(2)] = 2n(2) = 6 = 2q, q = 3.
b[n(k)] = 2n(k) = 2q^(k-1) = 2*3^(k-1),
n(k) = 3^(k-1)
