f(x)=log(2)x
f(an+1)-f(an)
=log(2)an+1-log(2)an
=log(2)an+1/an
=2
an+1/an=4
an为等比数列,公比4
根据通项公式可以得到an=a1q^n-1=2*4^(n-1)