A:extra
B:free
C:high
D:spare
Do you have a baseball bat?
Yes,____ ____.
(Ⅰ)求数列{an}的通项公式;
(Ⅱ)设 bn=nan ,Sn是数列{bn}的前n项和,对任意正整数n不等式 Sn+n2n+1>(−1)n⋅a 恒成立,求实数a的取值范围.