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问题: 高二数列2

已知数列{log3 (an-1)}(n∈N+)为等差数列,且a1=4,a3=28,求数列{an}的通项公式.

解答:

数列(Bn} ={log3(an-1)}, B1=log3(a1-1) =1, B3=3
==> Bn =n =log3(an-1)
==> an = 1+3^n