Sn=2an - n,S(n-1)=2a(n-1) - (n-1)
Sn-S(n-1)=an=2an - 2 a(n-1)-1 ,2a(n-1)=an -1
an +1= 2 [a(n-1)+1],(an +1)/[a(n-1)+1] = 2
所以(an +1)是公比为2的等比数列.
a1=2a1-1,a1＝1,a1 +1=2
an +1= 2*2^(n-1) = 2^n an = 2^n -1
bn=log2(an +1) = log2(2^n) =n