(1)
Sn =a1+a2+..+an
Vn = 2n+1
Sn/n = 2n+1
Sn = n(2n+1)(1)
S(n-1) = (n-1)(2n-1) (2)
(1)-(2)
an = n(2n+1) -(n-1)(2n-1)
= 2n^2 +n -(2n^2-3n+1)
=4n-1
(2)
an = 2^(n-1)
Sn = a1+a2+..+an
= 2^n-1
Vn = (2^n-1)/n
(Vn + 1/n).k >=3
(2^n). k >=3
k >= 3. 2^(-n)