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前几天的一个晚自习,我在办公室备课,两个同学拿着作业本来找我,他们为一道题的解法起了争执,两人都认为自己的解法是对的,但又都看不出对方的解法有什么错误,而两种解法的结果是不一样的,所以让我做裁判．题目很简单,是我们刚学过的数列的通项这一节课的课后作业题:已知数列{an}的首项为a1=3,且2an-1=Sn+1+Sn,求数列{an)的通项公式．下面是两位同学的解法．
A few days ago a night class study, I prepared lessons in the office, two classmates took homework to find me, they disputed the solution to a problem, both of them think that their solution is correct, but they can not see What’s wrong with the opponent’s solution, and the results of the two solutions are not the same, so let me be the referee. The title is very simple. It is the after-school assignment of the lesson we have just learned about the general term of the series: the first item of the known series {an} is a1=3, and 2an-1=Sn+1+Sn. The general formula of the series {an). Here are the solutions of the two classmates.