若二次根式 是最简二次根式，则最小的正整数a____．
She ________ his number in the phone book to make sure that she had got it right.
试回答下列问题：
是最简二次根式，则最小的正整数
为等差数列，其前n项和为
，已知
，若对任意
都有
成立，则k的值为（ ）