单选题
设数列 {an} 的前 n 项和为 Sn，则 {an} 等差。
(1) S n4=n 2+2n, n=1,2,3
(2) S n4=n 2+ 2n +1,n=1,2,3
题目答案
A
您的答案
答案解析
等差数列的前n项和公式为Sn=An2+Bn，A、B为全体实数，因此条件(1)充分，条件(2)不充分，选A。
举一反三
设圆 C 与圆 (x-5) 2+y2=2 ，关于 y=2x 时称，则圆 C 方程为 ( )
题目答案
E
您的答案
答案解析
暂无解析
Tom placed the bank notes ，_________the change and receipts ，back in the drawer ．
题目答案
D
您的答案
答案解析
暂无解析
I don ’t think you'll change his mind ；once he ’s decided on so something he tends to _____it.
题目答案
A
您的答案
答案解析
暂无解析
The team played hard because the championship of the state was______.
题目答案
B
您的答案
答案解析
暂无解析
In order to live the kind of life we want and to be the person we want to be ，we have to do more than just ________with events ．
题目答案
A
您的答案
答案解析
暂无解析
All her hard work __________in the end ，and she finally passed the exam ．
题目答案
B
您的答案
答案解析
暂无解析
The opportunity to explore and play and the encouragement to do so Can ________the performance of many children ．
题目答案
C
您的答案
答案解析
暂无解析
Successful learning is not a(n)________activity but consists of four distinct stages in a specific order
题目答案
C
您的答案
答案解析
暂无解析
We aim to ensure that all candidates are treated fairly and that they have equal______to employment opportunities ．
题目答案
C
您的答案
答案解析
暂无解析
Applicants should note that all positions are —— to Australian citizenship requirements ．
题目答案
A
您的答案
答案解析
暂无解析