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2又1/4+4又1/8+6又1/16+,…,+2n+1/2^(n+1) =(2+4+6+……+2n)+[1/4+1/8+1/16+……+1/2^(n+1)] =(2+2n)n/2+1/2-1/2^(n+1) =n(n+1)+1/2-1/2^(n+1)
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