GRE Prep Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.
Customized
for You
Track
Your Progress
Practice
Pays
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
In a sequence a1, a2,…, each term after the first is found
[#permalink]
Updated on: 27 Dec 2018, 03:24
Updated on: 27 Dec 2018, 03:24
1
Expert Reply
9
Bookmarks
00:00
Question Stats:
59% (01:36) correct
40% (02:22) wrong based on 226 sessions
Hide Show timer Statistics
In a sequence \(a_{1}\), \(a_{2}\),…, each term after the first is found by taking the negative of the preceding term, and adding 1. If \(a_{1} = 2\), what is the sum of the first 99 terms?
(A) 49
(B) 50
(C) 51
(D) 99
(E) 101
Re: In a sequence a1, a2,…, each term after the first is found
[#permalink]
19 Sep 2017, 09:43
19 Sep 2017, 09:43
2
1
Bookmarks
The sequence is 2, -1, 2, -1, ... and so on. Thus, every two elements of the sequence sum to 1. If we need to compute the sum of the first 99 terms, we can divide it by 2 in order to get the number of couples which sum to 1. 99/2=49.5, thus there are 49 couples so that 49*1=49, and we have the sum of the first 98 terms. The 99 term, being odd, will be equal to 2, thus we just have to sum 49 and 2 to get 51. Answer C!
Re: In a sequence a1, a2,…, each term after the first is found
[#permalink]
04 Feb 2018, 01:33
04 Feb 2018, 01:33
1
According to the questions,
a1=2
The sequence is a1+(-a1+1)+(a1-1+1)+.....
So we can get the sequence as follows:
2-1+2-1+... up to 99th term. So the answer is 49*2-49=49+2=51
So, C is the answer.
a1=2
The sequence is a1+(-a1+1)+(a1-1+1)+.....
So we can get the sequence as follows:
2-1+2-1+... up to 99th term. So the answer is 49*2-49=49+2=51
So, C is the answer.
