Find the sum SN of the first n terms of sequence 2,1 / 4,4,1 / 8,6,1 / 16,..., 2n + 1 / 2 ^ (n + 1)

2 1 / 4 + 4 1 / 8 + 6 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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