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M01-36: Is the product of a and b equal to 1?

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M01-36: Is the product of a and b equal to 1? [#permalink] New post 09 Jun 2012, 07:49
00:00
A
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C
D
E

Difficulty:

  5% (low)

Question Stats:

30% (01:39) correct 70% (00:47) wrong based on 10 sessions
Is the product of a and b equal to 1?
(1) a∗b∗a=a
(2) b∗a∗b=b

OA:
[Reveal] Spoiler:
Question: is ab=1?

(1) a^2*b=a → a^2*b−a=0 → a(ab−1)=0 → either a=0 (and b=any value, including zero) so in this case ab=0≠1 OR ab=1. Two different answers, not sufficient.

(2) a*b^2=b → a*b^2−b=0 → b(ab−1)=0 → either b=0 (and a=any value, including zero) so in this case ab=0≠1 OR ab=1. Two different answers, not sufficient.

(1)+(2) either a=b=0, so in this case ab=0≠1 and the answer to the question is NO, OR ab=1 and the answer to the question is YES. Two different answers, not sufficient.


I had figured that Each alone is insufficient.

But if we have two statements with only one common solution, then shouldnt that be the answer?
from 1 we get: either a=0 or ab=1
from 2 we get: either b=0 or ab=1
combining both we get only one common option, ab=1 - Sufficient
[Reveal] Spoiler: OA
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Re: M01-36: Is the product of a and b equal to 1? [#permalink] New post 14 Jun 2012, 02:57
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Expert's post
pavanpuneet wrote:
Hi Bunuel, we cant divide by 'a' in the first condition or for that fact 'b' in the second equation, as we do not know the sign of the a and b. Am i right?


Not quite. We cannot divide by a because a can be zero and division by zero is not allowed.

Never reduce equation by variable (or expression with variable), if you are not certain that variable (or expression with variable) doesn't equal to zero. We can not divide by zero. So, if you divide (reduce) a∗b∗a=a by a you assume, with no ground for it, that a does not equal to zero thus exclude a possible solution.

As for the sign: it becomes important with inequalities. For example if we have xy>x we cannot divide (reduce) by x since we don't know it's sign: if x>0 then we would have y>1 BUT if x<0 then we would have y<1 (flip the sign of the inequality when dividing/multiplying by negative value).

So, never multiply or divide inequality by a variable (or by an expression with variable) unless you are sure of its sign since you do not know whether you must flip the sign of the inequality.

Hope it's clear.
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Re: M01-36: Is the product of a and b equal to 1? [#permalink] New post 09 Jun 2012, 07:56
Expert's post
manulath wrote:
Is the product of a and b equal to 1?
(1) a∗b∗a=a
(2) b∗a∗b=b

OA:
[Reveal] Spoiler:
Question: is ab=1?

(1) a^2*b=a → a^2*b−a=0 → a(ab−1)=0 → either a=0 (and b=any value, including zero) so in this case ab=0≠1 OR ab=1. Two different answers, not sufficient.

(2) a*b^2=b → a*b^2−b=0 → b(ab−1)=0 → either b=0 (and a=any value, including zero) so in this case ab=0≠1 OR ab=1. Two different answers, not sufficient.

(1)+(2) either a=b=0, so in this case ab=0≠1 and the answer to the question is NO, OR ab=1 and the answer to the question is YES. Two different answers, not sufficient.


I had figured that Each alone is insufficient.

But if we have two statements with only one common solution, then shouldnt that be the answer?
from 1 we get: either a=0 or ab=1
from 2 we get: either b=0 or ab=1
combining both we get only one common option, ab=1 - Sufficient


Solution gives another possible case which satisfies both statements and gives a No answer to the question whether ab=1, namely a=b=0.
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: M01-36: Is the product of a and b equal to 1? [#permalink] New post 11 Jun 2012, 22:05
Is the product of a and b equal to 1?
(1) a∗b∗a=a
(2) b∗a∗b=b

1) not too sure, a could be 0
2) not too sure, b could be 0
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Re: M01-36: Is the product of a and b equal to 1? [#permalink] New post 14 Jun 2012, 02:44
Hi Bunuel, we cant divide by 'a' in the first condition or for that fact 'b' in the second equation, as we do not know the sign of the a and b. Am i right?
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Re: M01-36: Is the product of a and b equal to 1? [#permalink] New post 16 Jun 2012, 08:44
Rephrasing of the two statements yield
1) a=0 or ab=1 (not sufficient since ab=1 or ab=0)
2) b=0 or ab=1 (not sufficient since ab=1 or ab=0)

Combining yields same result, videlicit: ab=1 or ab=0

Thus E.

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Re: M01-36: Is the product of a and b equal to 1?   [#permalink] 16 Jun 2012, 08:44
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