题目

--Can I get you a cup of tea?     ---__________. A. Thank you for the tea                          B With pleasure C. You can , please                               D. That’s very nice of you 答案：D等差数列{an}中,a7=4,a19=2a9.(1)求{an}的通项公式;(2)设bn=,求数列{bn}的前n项和Sn.