解:
n≥2时,an=3a(n-1)+3ⁿ-1
an-½=3a(n-1)- 3ⁿ -3/2
an-½=3[a(n-1)-½] -3ⁿ
等式两边同除以3ⁿ
(an-½)/3ⁿ=[a(n-1)-½]/3ⁿ⁻¹-1
(an-½)/3ⁿ-[a(n-1)-½]/3ⁿ⁻¹=1,为定值
(a1-½)/3=(5-½)/3=3/2
数列{(an-½)/3ⁿ}是以3/2为首项,1为公差的等差数列
(an-½)/3ⁿ=(3/2) +1·(n-1)=n+½
an=(n+½)·3ⁿ+½
n=1时,a1=(1+½)·3+½=5,同样满足表达式
数列{an}的通项公式为an=(n+½)·3ⁿ+½