Sequence

Expert replies

Previous Topic Next Topic

mehravikas: Legendary Member; Posts: 1161; Joined: Mon May 12, 2008 2:52 amFollowed by: 1 member; Location: Sydney; Thanked: 23 times

by mehravikas » Sun Oct 05, 2008 5:32 pm

Got an answer by process of elimination. Please help in getting the correct funda.

OA is: A

Attachments

Like this post

Source: Beat The GMAT — Problem Solving | Sun Oct 05, 2008 5:32 pm

cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Sun Oct 05, 2008 5:39 pm

The answer should be A)

X1 = 3 (2^1+1)
X2 = 5 (2^2+1)
X3 = 9 (2^3+1)

....

THEREFORE X20 = 2 ^ 20 + 1
X19 = 2^19+1

X20-X19
= 2^20+1 - (2^19+1)
= 2^20 + 1 - 2^19 - 1
= 2 ^ 20 - 2 ^ 19

TAKING COMMON FACTOR 2^19 OUT
2^19(2 ^ 1 - 1)
= 2 ^ 19

Like this post

cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Sun Oct 05, 2008 5:41 pm

Just saw the OA and its A)

Like this post

cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Sun Oct 05, 2008 5:43 pm

My thought is on these type of problems find a pattern for the terms in the sequence as calculating it manually would take more than 2 minutes to solve.

For eg: In the problem above expressing each term in terms of 2 ^ something

Like this post

4meonly: Legendary Member; Posts: 891; Joined: Sat Aug 16, 2008 4:21 amFollowed by: 1 member; Thanked: 27 times; GMAT Score:660(

by 4meonly » Mon Oct 06, 2008 9:27 am

cramya wrote:The answer should be A)

X1 = 3 (2^1+1)
X2 = 5 (2^2+1)
X3 = 9 (2^3+1)

I dont understand how you made it from the question without any powers?
I question is not correct, could you please post in correct manner?
thanx!

Like this post

cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Mon Oct 06, 2008 2:11 pm

Hi 4meonly,

X1 = 3
X2 = 5
X3 = 9

I was saying that the first term X1 which is 3 can be expressed as 2 ^1 + 1
2nd term x2 which is 5 can be expressed as 2 ^2+1

Similarly
x19 can be expressed as 2 ^ 19 + 1
x20 can be expressed as 2 ^ 20 +1

Hope this helps!

Like this post

sudhir3127: Legendary Member; Posts: 829; Joined: Mon Jul 07, 2008 10:09 pmFollowed by: 3 members; Location: INDIA; Thanked: 84 times

by sudhir3127 » Sun Nov 30, 2008 12:15 am

cramya wrote:The answer should be A)

X1 = 3 (2^1+1)
X2 = 5 (2^2+1)
X3 = 9 (2^3+1)

....

THEREFORE X20 = 2 ^ 20 + 1
X19 = 2^19+1

X20-X19
= 2^20+1 - (2^19+1)
= 2^20 + 1 - 2^19 - 1
= 2 ^ 20 - 2 ^ 19

TAKING COMMON FACTOR 2^19 OUT
2^19(2 ^ 1 - 1)
= 2 ^ 19

Nice way of solving ... but if u wanna do it real quick u can cut down on most of the steps...

X1 = 3
X2 = 5
X3 = 9

now X2- X1 = 2(2^1)
X3- X1 = 4(2^2)

2,4,8,..............

thus X20 - X 19 = 2^19.

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