等差数列{an}和{bn}的前n项和分别为Sn和Tn，且SnTn=2n3n+1，

搜题请搜索小程序“爱下问搜题”

问题填空题

等差数列{a_n}和{b_n}的前n项和分别为S_n和T_n，且

3n+1

，则

=______．

答案

由

3n+1

n(a1+an)

n(b1+bn)

a1+an

b1+bn

，

而

(a1+a9)

(b1+b9)

a1+a9

b1+b9

即当n=9时，

3n+1

故答案为

选择题

25. ______, a small advertisement held my attention, which read “Easy job. Good wages. No

experience necessary.”

A．Looking through the newspaper

B．While I was looking through the newspaper

C．To look through the newspaper

D．I was looking through the newspaper

判断题

有连皮的锻件，冷锻件图上不要绘出连皮的形状和尺寸，而在热锻件图上要绘出连皮的形状和尺寸。（）