d=(21-9)/3=4
a1=5
an=a1+(n-1)*d=4n+1
bn=2^(4n+1)
bn-1=2^[4(n-1)+1]=2^(4n-3)
bn+1=2^[4(n+1)+1]=2^(4n+5)
bn+1/bn=2^(4n+5)/2^(4n+1)=2^4=16
bn/bn-1=2^(4n+1)/2^(4n-3)=2^4=16
所以,数列{bn}是等比数列