已知数列｛an｝满足a1=1,an=3^n-1+an-1,证明an=3^n-1/2我已经知道an=3+3^2+.+3^(n-1)+a1接着是an=3(1-3^(n-1))/(1-3)+a1=3^n-1/2不过不知道怎么下来,只有今天,有没有更好的回答弄

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