数列{an}满足=3an-1+3^n-1,(n≥2),a4=365,an的前n项之和为Sn,求Sn

题目

数列{an}满足=3an-1+3^n-1,(n≥2),a4=365,an的前n项之和为Sn,求Sn

答案

a1=5
an=-3^n/2+2 (n≥2)