f(2^an)=log2(2^an)-log(2^an)(4)=an-2/an=2n
an^2-2nan-2=0
(an-n)^2=n^2+2
an=n+√(n^2+2)或an=n-√(n^2+2)
an=n+√(n^2+2)时,an随n增大单调递增,n=1时最小,a1=1+√3
an=n-√(n^2+2)=-2/[n+√(n^2+2)],分母n+√(n^2+2)单调递增,an单调递增,n=1时最小,a1=1-√3