BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem
TTP OnDemand GMAT Video Course
Featured Course

Target Test Prep GMAT OnDemand

Scott Woodbury-Stewart’s private virtual classroom — 400 hours of master-class video lessons for the GMAT Focus Edition.

  • 715+ score guarantee — highest in the industry (99th percentile)
  • 52 chapters · 1,500+ lessons · 4,000+ practice questions
  • 400 hours of video · 1,500+ instructor-led HD smartboard lessons
  • 300,000+ students accepted to Harvard, Stanford, Wharton, Booth & Sloan & more
  • 24/7 live support + weekly Zoom office hours with GMAT instructors
  • TTP AI Assist — 24/7 AI-powered virtual tutor for instant help
  • 1,200+ flashcards + AI-powered study assistant & daily calendar
  • OnDemand, LiveTeach & GMAT Bootcamp formats available
  • Also: GRE, SAT Math & Executive Assessment courses
  • MBA Admissions Consulting now available
  • 🏆 2025 EdTech Breakthrough Award: Test Prep Solution Provider of the Year
  • 200,000+ students served
  • 5-day free trial — $0 to start, no auto-billing, cancel anytime
★★★★★ 5.0 (559 reviews)
130-pt guarantee
$0 to start then $127/mo
Start Free 5-Day Trial Read 559 Reviews →

Kaplan DS

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 40
Joined: Mon Sep 24, 2007 1:27 pm
by davy420 » Sun Oct 19, 2008 7:58 pm
Can somebody help me understand this question

What is the value of A-B

1) A^2 - B^2 = 9

2) A^2 - 2AB + B^2 =1

The answer is E
But I thought the answer is D
because for statment 1 if you square root both sides A-B = 3
for statement 2
(A-B) (A-B) = 1
Am I doing something wrong .thanks
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Data Sufficiency |
User avatar
Senior | Next Rank: 100 Posts
Posts: 31
Joined: Tue Jul 22, 2008 9:09 am
Location: Toronto
Thanked: 7 times
GMAT Score:770

by madeline » Sun Oct 19, 2008 8:26 pm
(1)
Square root of A^2 - B^2 is NOT equal to A-B! In fact, it has nothing to do with A-B.
As you do know yourself, (A-B)(A-B) = A^2 - 2AB + B^2, not simply A^2 - B^2

(2) A^2 - 2AB + B^2 =1
From this we can deduce either of the following:
(A-B)(A-B) = 1
or (B-A)(B-A) = 1
If the former is true, then A-B=1; if the latter is true, then A-B=-1
So statement 2 alone is not sufficient either.

Looking at the 2 statements together, you can play around a little but you will not arrive at a definite answer for A-B.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Senior | Next Rank: 100 Posts
Posts: 40
Joined: Mon Sep 24, 2007 1:27 pm

by davy420 » Sun Oct 19, 2008 9:14 pm
Sorry but I am still confused

I understand that (A-B)(A-B) = A^2 - 2AB + B^2 but doesn't
(A-B)(A-B) = (A-B)^2 or A^2 - B^2 if you simplifed it. Sorry im still confused. thanks
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Legendary Member
Posts: 871
Joined: Wed Aug 13, 2008 7:48 am
Thanked: 48 times

by stop@800 » Mon Oct 20, 2008 4:29 am
davy420 wrote:Sorry but I am still confused

I understand that (A-B)(A-B) = A^2 - 2AB + B^2 but doesn't
(A-B)(A-B) = (A-B)^2 or A^2 - B^2 if you simplifed it.
Yes
(A-B)(A-B) = (A-B)^2
but
(A-B)^2 != A^2 - B^2

rather
(A-B)^2 = A^2 + B^2 - 2AB

You can verify by multiplying (A-B) with (A-B)
Sorry im still confused. thanks
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 2467
Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members
Thanked: 331 times

by cramya » Mon Oct 20, 2008 5:06 am
(1)a^2-b^2 = (a+b) (a-b)

a+b = 4
a-b= 9/4

a=25/8
b= 7/8

or
a + b = 3
a - b = 3

a=3 b=0

We cannot come up with a definite value for a-b INSUFFICIENT

For (2)

(A-B) (A-B) = 1

A-B CAN BE EQUAL TO -1 OR 1

INSUFFICIENT

Taking both together still INSUFFICIENT(we cannot come to a definite value
HENCE E
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Senior | Next Rank: 100 Posts
Posts: 65
Joined: Sun Oct 19, 2008 2:18 pm
Thanked: 1 times

Re: Kaplan DS

by yezz » Mon Oct 20, 2008 6:00 am
davy420 wrote:Can somebody help me understand this question

What is the value of A-B

1) A^2 - B^2 = 9

2) A^2 - 2AB + B^2 =1
from 1

(a-b)(a+b) = 9 thus a-b can = -3 0r 3...insuff

from 2
(A-B)(A-B) = 1 a-b can =1 or -1...insuff

both

no common roots...E
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Senior | Next Rank: 100 Posts
Posts: 40
Joined: Mon Sep 24, 2007 1:27 pm

by davy420 » Mon Oct 20, 2008 5:12 pm
ohhh ok i understand now. Thanks everyone for the explaination
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1