题目

---Hello, May I speak to Tom? ---__________.        A. Hold the line please.                        B. I am Tom.        C. Who are you?                                       D. Are you Linda? 答案：A已知数列{an}的前n项和为Sn，且满足an+Sn=2．（1）求数列{an}的通项公式；（2）求证数列{an}中不存在三项按原来顺序成等差数列．