题目

—Hurry up! Alice and Sue are waiting for you at the school gate.—Oh!I thought they (go) without me. 答案：【1】would go已知数列{an}满足a1＞0，an+1=2－，。（1）若a1，a2，a3成等比数列，求a1的值；（2）是否存在a1，使数列{an}为等差数列？若存在，求出所有这样的a1，若不存在，说明理由。