题目

阅读下列短文，从每题所给的四个选项（A、B、C和D）中，选出最佳选项。Dear friends, The recent success of children's books has made the general public aware that there's a huge market out there. And there's a growing need for new writers trained to create the $3 billion worth of children's books bought each year… plus stories and articles needed by over 650 publishers of magazines for children and teenagers. Who are these needed writers? They're ordinary people like you and me. But am I good enough? I was once where you might be now. My thoughts of writing had been14．已知数列{an}的前n项和为Sn且满足Sn+an=2n．（1）写出a1，a2，a3，并推测an的表达式；（2）用数学归纳法证明所得的结论．