【单选题】对数列{a n },若存在正常数M,使得对任意正整数n,都有|a n |<M,则称数列{a n }是有界数列.下列三个数列: a n = 1 3 (1- 2 n ) ; a n = 2 n +3 2 n -3 ; a n =( 1 4 ) n -( 1 2 ) n 中,为有界数列的个数是( )
【单选题】设数列{ }的前n项和为S n (n∈N),关于数列{ }有下列四个命题: (1)若{ }既是等差数列又是等比数列,则a n =a n +1 (n∈N * ); (2)若S n =An 2 +Bn(A,B∈R,A、B为常数),则{ }是等差数列; (3)若S n =1-(-1) n ,则{ }是等比数列; (4)若{ }是等比数列,则S m ,S 2m -S m ,S 3m -S 2m (m∈N...