题目

Is her birthday (in/on) July? 答案：【1】in17．下列四个函数中偶函数的序号为①④①$f（x）=\root{3}{x^2}+1$②$f（x）=x+\frac{1}{x}$③$f（x）=\sqrt{1+x}-\sqrt{1-x}$④f（x）=x2+x-2．