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規律為:首項為1,等比為3的等比數列前n項之和 囙此,第n項為: 1*(1-3^n)/(1-3)=(3^n-1)/2
4-1=3 13-4=9 40-13=27 81-27=54你看看有啥規律 或許最大的規律就是他永遠也沒個規律.
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