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小弟我基础较弱,

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小弟我基础较弱,
小弟我基础较弱,
原式=3+[1-(1/2)^(n-1)]/(1/2)-[(2n+1)/(2^n)]
=3+2[1-2^(1-n)]-[(2n+1)/(2^n)]
=3+2-2*2^(1-n)-[(2n+1)/(2^n)]
=5-2^(2-n)-[(2n+1)/(2^n)]
=5-[2²/(2^n)]-[(2n+1)/(2^n)]
=5-[4/(2^n)]-[(2n+1)/(2^n)]
=5-[4+(2n+1)]/(2^n)
=5-[(2n+5)/(2^n)]