已知数列{an}满足a1=3,(an+1)-3an=3^n(n,n∈N*),数列{bn}满足bn=3^(-n)an求证：数列{bn}是等差数列设sn=(a1)/3+(a2)/4+(a3)/5+.(an)/(n+2),求满足1、128<sn/s2n<1/4的所有正整数n的值

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