题目

What is Nancy doing at 4: 00 p. m.? (看图回答问题) _______________________________________________________________ 答案：【答案】She is reading a book.【解析】句意：Nancy在下午4点做什么？图片可知是在看书。句子时态为现在进行时，故答案为She is reading a book.已知数列{an}是等差数列，且a1=2，a1+a2+a3=12．（1）求数列{an}的通项公式；（2）求数列{an}的前n项和Sn．