题目

When May hurried home, she found that her books in the classroom. A . has been left B . had left C . has left D . had been left 答案：D20．已知函数f（x）=x+$\frac{k}{x}$，其中k为常数，f（1）=5．（1）求函数f（x）的解析式；（2）求证：f（x）是奇函数；（3）求证：f（x）在区间（0，2）上是减函数．