题目

—Welcome back to school, Tim. I want to know ___________online.—Of course, Ms. Clark, I was hard-working. A . what you studied B . when you studied C . if you studied hard 答案：C19．数列{an}的通项公式为an=4n-1，（1）求数列{an}前n项的和为Sn；（2）令bn=$\frac{S_n}{n}$，求数列{2nbn}的前n项的和Tn．