已知数列{an}的前n项和Sn=2n-1，则{an}（） A．一定是等差数列_爱下问搜题

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问题选择题

已知数列{a_n}的前n项和Sn=2n-1，则{a_n}（　　）

A．一定是等差数列

B．一定是等比数列

C．或是等差数列，或是等比数列

D．既不是等差数列，也不是等比数列

答案

∵S_n=2ⁿ-1，

当n=1时，a₁=S₁=1，

当n≥2时，a_n=S_n-S_n-1=（2ⁿ-1）-（2^n-1-1）=2^n-1，

显然，n=1时a₁=1符合上式．

∴a_n=2^n-1．

∵

an+1

an

=

2n

2n-1

=2，a₁=1，

∴数列{a_n}是以1为首项，2为公比的等比数列．

故选B．

单项选择题

急性心肌梗死分为“ST段抬高型”和“非ST段抬高型”心肌梗死的临床意义是（）。

A.有助于判断单支还是多支冠状动脉病变

B.有助于判断心功能的状态

C.有助于早期进行干预治疗并选择正确的治疗方案

D.有助于判断心内膜下梗死还是透壁梗死

E.以上都对

完形填空

My sister and I have been a headache for my mother, because my father was in the army, he was not around too often. __21__, my mother brought us up almost single-handedly and it must have __22__ real strength of mind to bring up such naughty and stubborn children who got out from one trouble only to get into another.
Bonus and I weren’t scholars either. __23__, we hated studying from the very beginning. We were __24__ to spend time with our schoolbooks---out of the fear of Mum’s scolding. I almost did not make it to 10th grade. That entire summer Mum __25__ that I read my schoolbooks from the very first chapter till the last.
I left __26__ college at 18. In my second year of college, while I was studying for my final exams, I got a phone call from Mum __27__ my sister had failed her 9th grade exams. Mum was heartbroken. My mother was an English teacher in a famous school in the city and it was a(n) __28__ for her.
Then started another __29__ for my Mum. There always seemed to be people who would come and talk about how well their children were doing __30__ my mother would be having a tough time making sure we stayed on the right path. Mum was __31__ that we would do well for ourselves. She never let my sister and me __32__ that we had lost out on something and always told us: “Let this be a lesson. We are going to work really really hard...__33__ will change.”
Things did change, but very slowly and __34__, one little step at a time. Last year I finished with law school and started working. My mother’s belief in me __35__. And just a few days ago I got a phone from Bonus. She had __36__ the exam with honors and was __37__ to the best university in the country. I was in tears. Later that night I __38__ Mum: “Mum, your two little failures didn’t do as badly as everybody thought.” Mum __39__ and said: “You are my daughters and I knew you the best. I always knew that you would succeed. It was just a __40__ of time.”

A．In that case

B．In this way

C．On the other hand

D．On the contrary

A．Environment

C．Circumstances

D．Impressions