Find all School-related info fast with the new School-Specific MBA Forum

 It is currently 25 Oct 2016, 14:37

### GMAT 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

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# Find the sum of the first 15 terms of the series whose nth

Author Message
TAGS:

### Hide Tags

Intern
Joined: 22 Sep 2011
Posts: 28
Followers: 0

Kudos [?]: 21 [0], given: 20

Find the sum of the first 15 terms of the series whose nth [#permalink]

### Show Tags

14 Oct 2013, 09:34
3
This post was
BOOKMARKED
00:00

Difficulty:

25% (medium)

Question Stats:

77% (02:17) correct 23% (01:45) wrong based on 87 sessions

### HideShow timer Statistics

Find the sum of the first 15 terms of the series whose nth term is (4n+1).

A. 485
B. 495
C. 505
D. 630
[Reveal] Spoiler: OA

Last edited by Bunuel on 30 Oct 2013, 02:54, edited 2 times in total.
Renamed the topic and edited the question.
Verbal Forum Moderator
Joined: 10 Oct 2012
Posts: 630
Followers: 78

Kudos [?]: 1054 [2] , given: 136

Re: Find the sum of the first 15 terms of the series whose nth [#permalink]

### Show Tags

14 Oct 2013, 09:42
2
KUDOS
1
This post was
BOOKMARKED
jrymbei wrote:
Find the sum of the first 15 terms of the series whose nth term is (4n+1).

A. 485
B. 495
C. 505
D. 630

First term : 4*1+1 = 5.

15th term : 4*15+1= 61

Sum of 15 terms : $$\frac{No of terms*(first term + last term)}{2} = \frac{15*(61+5)}{2} = 495.$$

B.
_________________
Intern
Joined: 22 Sep 2011
Posts: 28
Followers: 0

Kudos [?]: 21 [0], given: 20

Re: Find the sum of the first 15 terms of the series whose nth [#permalink]

### Show Tags

14 Oct 2013, 12:07
Thanks for the explanation...it was so simple!!!
Intern
Joined: 14 Dec 2011
Posts: 18
Location: India
Concentration: Technology, Nonprofit
GMAT 1: 640 Q48 V29
GMAT 2: 660 Q45 V35
GPA: 3.5
WE: Information Technology (Computer Software)
Followers: 0

Kudos [?]: 19 [0], given: 77

Re: Find the sum of the first 15 terms of the series whose nth [#permalink]

### Show Tags

30 Oct 2013, 02:53
jrymbei wrote:
Find the sum of the first 15 terms of the series whose nth term is (4n+1).

A. 485
B. 495
C. 505
D. 630

Formula used: Sum of n terms = Average (First and Last term) * Number of terms.

First term : n=1, (4*1+1) = 5
Last term : n=15, (4*15 + 1) = 61

Sum = (5+61)/2 * 15 = 495. Answer B.
Intern
Joined: 01 Feb 2015
Posts: 5
Followers: 0

Kudos [?]: 0 [0], given: 8

Find the sum of the first 15 terms of the series whose nth [#permalink]

### Show Tags

26 Mar 2015, 22:30
Its probably obvious.. but can someone explain why the formula for a arithmetic series is used here ... Sum of series n = (n/2)(A1 - An) ... as opposed to the formula for a geometric series ... Sum of series n = [A1*(1-r^n)]/(1-r) where r is the common ratio and n is the nth term.

I understand the solution but am confused by the formula thinking that An=4n+1 is a geometric sequence given that you have to multiply by 4 to get the next term in the sequence. Appreciate your reply!!

THANKS!
Math Expert
Joined: 02 Sep 2009
Posts: 35286
Followers: 6644

Kudos [?]: 85673 [0], given: 10242

Re: Find the sum of the first 15 terms of the series whose nth [#permalink]

### Show Tags

27 Mar 2015, 04:31
sisorayi01 wrote:
Its probably obvious.. but can someone explain why the formula for a arithmetic series is used here ... Sum of series n = (n/2)(A1 - An) ... as opposed to the formula for a geometric series ... Sum of series n = [A1*(1-r^n)]/(1-r) where r is the common ratio and n is the nth term.

I understand the solution but am confused by the formula thinking that An=4n+1 is a geometric sequence given that you have to multiply by 4 to get the next term in the sequence. Appreciate your reply!!

THANKS!

The sequence is define by $$a_n=4n+1$$, thus:

$$a_1=4*1+1=5$$;
$$a_2=4*2+1=9$$;
$$a_3=4*3+1=13$$;
$$a_4=4*4+1=17$$;
...

As you can see the sequence we have (5, 9, 13, 17, ...) is an arithmetic progression, not geometric.
_________________
Intern
Joined: 17 Feb 2015
Posts: 28
GPA: 3
Followers: 0

Kudos [?]: 29 [0], given: 13

Re: Find the sum of the first 15 terms of the series whose nth [#permalink]

### Show Tags

27 Mar 2015, 05:24
sisorayi01 wrote:
Its probably obvious.. but can someone explain why the formula for a arithmetic series is used here ... Sum of series n = (n/2)(A1 - An) ... as opposed to the formula for a geometric series ... Sum of series n = [A1*(1-r^n)]/(1-r) where r is the common ratio and n is the nth term.

I understand the solution but am confused by the formula thinking that An=4n+1 is a geometric sequence given that you have to multiply by 4 to get the next term in the sequence. Appreciate your reply!!

THANKS!

Adding to what Bunuel just posted, a geometric series would look like following
An = 4^n
And the series would look something like 4, 16, 64, 256, 1024, ...
Re: Find the sum of the first 15 terms of the series whose nth   [#permalink] 27 Mar 2015, 05:24
Similar topics Replies Last post
Similar
Topics:
1 In a certain series, each term (except for the first term) is one grea 1 14 Jun 2015, 05:09
12 What is the tens' digit of the sum of the first 40 terms of 7 26 Jan 2012, 07:28
28 The second, the first and the third term of an AP whose comm 17 08 Dec 2011, 08:15
1 Find the 28383rd term of series 123456789101112..... 4 04 Nov 2009, 01:01
15 Find the the sum of the first 20 terms of this series which 21 02 Dec 2006, 20:03
Display posts from previous: Sort by