数列an满足an=3an-1+3的n次方-1（n大于等于2）.又a1=5,则使得(an+b)除以3的n次方为等差数列则b是多少

a(n+1)=3a(n) + 3^(n+1) - 1 ,a(n+1) - 1/2 = 3a(n) + 3^(n+1) - 3/2 = 3[a(n)-1/2] + 3^(n+1),[a(n+1)-1/2]/3^(n+1) = [a(n)-1/2]/3^(n) + 1,{[a(n)-1/2]/3^n}是首项为[a(1)-1/2]/3=3/2,公差为1的等差数列.b=-1/2....