问题
选择题
— Are you reading?
— No, ___________.[ ]
A. it isn't
B. it is
C. I am
D. I'm not
答案
答案：D
搜题请搜索小程序“爱下问搜题”
— Are you reading?
— No, ___________.[ ]
A. it isn't
B. it is
C. I am
D. I'm not
答案：D
的前n项和，那么Tn=（）。