a 1= 1，q=2

an=a 1q？ = 1 2? =2?

bn=3+log2(an)

=3+log2(2？ )

=3+(n- 1)

=n+2

b 1= 1+2=3

B (n+1)-bn = [(n+1)+2]-(n+2) =1,which is a fixed value.

Sequence {bn} is a arithmetic progression, the first term is 3, and the tolerance is 1.