Different values thru the geometric progression?

Expert replies

ov25: Master | Next Rank: 500 Posts; Posts: 201; Joined: Sat Jul 10, 2010 2:23 pmFollowed by: 1 member; Thanked: 7 times

by ov25 » Fri Dec 17, 2010 4:48 pm

For the geometric sequence, a1, a1*d, a1*d^2, ... a1*d^n, the sum of the first three terms is
21, what is the sum of the first 6 terms?
(1) a1=3
(2) d>0

discuss

Like this post

Source: Beat The GMAT — Data Sufficiency | Fri Dec 17, 2010 4:48 pm

Night reader: Legendary Member; Posts: 1337; Joined: Sat Dec 27, 2008 6:29 pmFollowed by: 10 members; Thanked: 127 times

by Night reader » Fri Dec 17, 2010 5:28 pm

ov25 wrote:For the geometric sequence, a1, a1*d, a1*d^2, ... a1*d^n, the sum of the first three terms is
21, what is the sum of the first 6 terms?
(1) a1=3
(2) d>0

discuss

a1*d(1+d)+a1=21, to find the sum of the first six we need to find d
st(1) a1=3 <=> 3*d(1+d) +3=21 <=> 3d+3*d^2=18 <=> d^2+d-6=0 <=> (d+3)(d-2)=0 => 2 answers for d {-3:2} Not sufficient
st(2) d>0. Not sufficient.

Combining st(1&2) d=2 Sufficient.

IOM C

Like this post

Bharat: Senior | Next Rank: 100 Posts; Posts: 70; Joined: Sun Jul 22, 2007 11:43 pm; Thanked: 8 times

by Bharat » Fri Dec 17, 2010 5:38 pm

Answer: C (both together are sufficient).

as per given condition: a + ad + ad^2 = 21 (=S3)
a*(1 + d + d^2) = 21
Hence To calculate the sum of 6 terms, we need to know both a & d.
lets check the given conditions:

I: a = 3: it means: (1+ d+ d^2) = 7
solve to get: (d+3)(d-2) = 0 -> d = -3, 2 NOT SUFFICIENT.

II: d > 0: NOT SUFFICENT (value of a is still wide open)

Take I & II together: d = 2, a = 3 SUFFICIENT. Hence (D)

Let me know if you have questions.
Regards,
Bharat.

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review