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Kaplan DS

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
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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.
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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
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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
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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
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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
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by davy420 » Mon Oct 20, 2008 5:12 pm
ohhh ok i understand now. Thanks everyone for the explaination
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