ebw20100
幼苗
共回答了8个问题采纳率：87.5% 举报
令S=b1+b2+b3+.+bn
=1/2^1+2/2^3+3/2^5+.n/2^(2n-1)
s/4=1/2^3+2/2^5+3/2^7+.n/2^(2n+1)
s-s/4=1/2^1+1/2^3+1/2^5+.+1/2^(2n-1)-n/2^(2n+1)
3s/4=1/2+1/8*[(1-1/4^(n-1)/(1-1/4)]-n/2^(2n+1)
s=2/3+2/9*[(1-1/4^(n-1)]-n/2^(2n+1)
1年前
追问
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