sn=3an-3^(n+1)
sn-1 =3an-1 -3^(n)
两式相减 an=3an-3an-1 - 3^(n+1)- 3^n
整理得 an/3^n= 1/2*( an-1/3^(n-1)) + 1
所以设 an/3^n=bn
则bn=1/2bn-1+1
bn -2= 1/2 (bn-1 -2)
a1=3a1-9 a1=9/2 b1-2=9/3*2-2=-1/2
bn -2=-1/2*(1/2)^n-1
an=2*3^n-(3/2)^n
sn=3an-3^(n+1)
sn-1 =3an-1 -3^(n)
两式相减 an=3an-3an-1 - 3^(n+1)- 3^n
整理得 an/3^n= 1/2*( an-1/3^(n-1)) + 1
所以设 an/3^n=bn
则bn=1/2bn-1+1
bn -2= 1/2 (bn-1 -2)
a1=3a1-9 a1=9/2 b1-2=9/3*2-2=-1/2
bn -2=-1/2*(1/2)^n-1
an=2*3^n-(3/2)^n