sn=2*1/2+4*1/2^2+6*1/2^3+

sn=2*1/2+4*1/2^2+6*1/2^3+……+2n*1/2^n

1个回答

sn=2*1/2+4*1/2^2+6*1/2^3+……+2（n-1）*1/2^(n-1)+2n*1/2^n
所以
s（n-1）=2*1/2+4*1/2^2+6*1/2^3+……+2（n-1）*1/2^(n-1)
an=sn-s(n-1)=2n*1/2^n

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