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If x is an integer, is x|x| < 2^x?

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If x is an integer, is x|x| < 2^x? [#permalink] New post 17 Jan 2007, 15:27
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If x is an integer, is x|x| < 2^x?

(1) x < 0
(2) x = -10
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 [#permalink] New post 17 Jan 2007, 19:03
1) If x<0, then x*|x| is always negative. 2^x is positive. So, this statement is sufficient.

2) Just plug it in. Sufficient.

Answer is D.
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 [#permalink] New post 17 Jan 2007, 19:04
D for me

I couldn't figure out using statement 1 so I ignored it to start with. Using statement 2, x|x| will always be < 2^x. So the answer is either B or D.

Going back to statement 1, I plugged in negative values such as -1, -2 and also fractions and realised that if x is not an integer, the result need not always be true but if x is an integer, x|x| will always be < 2^x. And the question stem does say that x is an integer.

Hence D for me.
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 [#permalink] New post 19 Jan 2007, 01:08
If you determine that A satisfies the question then B would have to also.
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DS Questions - absolute integers - Help understand logic [#permalink] New post 13 Sep 2010, 15:41
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If X is an integer is X |x| < 2^X

1. X<0
2. X=-10

I solved it - using two options
-10 |10| < 1/2^10 AND -10 -|10| < 1/2^10. This method gives two solutions and therefore not sufficient. However my logic is wrong. Please explain why there are not two options. I have come across questions where one is required to use the two options. why not in this case? thanks
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Re: DS Questions - absolute integers - Help understand logic [#permalink] New post 13 Sep 2010, 15:56
Expert's post
cmugeria wrote:
If X is an integer is X |x| < 2^X

1. X<0
2. X=-10

I solved it - using two options
-10 |10| < 1/2^10 AND -10 -|10| < 1/2^10. This method gives two solutions and therefore not sufficient. However my logic is wrong. Please explain why there are not two options. I have come across questions where one is required to use the two options. why not in this case? thanks


If x is an integer, is x*|x|<2^x

This is YES/NO data sufficiency question: In a Yes/No Data Sufficiency question, each statement is sufficient if the answer is “always yes” or “always no” while a statement is insufficient if the answer is "sometimes yes" and "sometimes no".

Now, you should notice that the RHS (right hand side) of the expression is always positive (2^x>0), but the LHS is positive when x>0 (x>0 --> x*|x|=x^2), negative when x<0 (x<0 --> x*|x|=-x^2) and equals to zero when x={0}.

(1) x<0 --> according to the above x*|x|<0<2^x, so the answer to the question "is x*|x|<2^x" is YES. Sufficient.

(2) x=-10, the same thing here x*|x|=-100<0<\frac{1}{2^{10}}, so the answer to the question "is x*|x|<2^x" is YES. Sufficient.

Answer: D.


cmugeria wrote:
-10 |10| < 1/2^10 AND -10 -|10| < 1/2^10.


When x=-10 then |x|=|-10|=10 and x*|x|=-10*10=-100.

Hope it's clear.
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Re: DS Questions - absolute integers - Help understand logic [#permalink] New post 13 Sep 2010, 17:20
To give you another way of looking at it...

In an inequality you can't move variables from side to side using multiplication or division unless you know their sign since the inequality expression will change direction if you multiply or divide both sides by a negative number.

Both statements tell you that X is negative, so you know that you can now divide both sides by X, giving you a definite equation that |x| > (2^x)/X
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Re: DS Questions - absolute integers - Help understand logic [#permalink] New post 13 Sep 2010, 18:08
Thank you for the explanation.

Please help me understand what the difference (in regards to having two solutions in terms of absolute and non absolute values) is between the two questions
is x*|x|<2^x and the question
|x+1|= x*|3x-2|what are the possible values for x from advanced equations of MGMAT Equations and inequalities - the answer is 1/4 and 3/2

Maybe i am overanalyzing the questions
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Re: DS Questions - absolute integers - Help understand logic [#permalink] New post 13 Sep 2010, 18:45
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cmugeria wrote:
Thank you for the explanation.

Please help me understand what the difference (in regards to having two solutions in terms of absolute and non absolute values) is between the two questions
is x*|x|<2^x and the question
|x+1|= x*|3x-2|what are the possible values for x from advanced equations of MGMAT Equations and inequalities - the answer is 1/4 and 3/2

Maybe i am overanalyzing the questions


I don't quite understand your question.

Original question asks whether x*|x|<2^x is true, it's YES/NO DS question, it doesn't ask for specific value of x. AGAIN: In a Yes/No Data Sufficiency question, each statement is sufficient if the answer is “always yes” or “always no” while a statement is insufficient if the answer is "sometimes yes" and "sometimes no". As EACH statement ALONE gives the definite answer YES x*|x| is less than 2^x then EACH statement ALONE is sufficient to answer the question which means than answer is D .

Another one |x+1|=|3x-2| (I believe it's |x+1|=|3x-2| and not |x+1|= x*|3x-2| as you wrote, as solutions you provided 1/4 and 3/2 satisfy the first equation and not the second one), seems to be another type of DS question, the one which asks for a certain value of an unknown. For this type of questions statement is sufficient if it gives single numerical value of this unknown. So as |x+1|=|3x-2| has two solutions x=\frac{1}{4} and x=\frac{3}{2} then this statement (if this is the only thing we know for certain statement) is not sufficient, as it does not give single numerical value of x.

For more absolute values and inequalities:

Check Walker's topic on ABSOLUTE VALUE: math-absolute-value-modulus-86462.html

For practice check collection of 13 tough inequalities and absolute values questions with detailed solutions at: inequality-and-absolute-value-questions-from-my-collection-86939.html

700+ PS and DS questions (also have some inequalities and absolute values questions with detailed solutions):
tough-problem-solving-questions-with-solutions-100858.html

700-gmat-data-sufficiency-questions-with-explanations-100617.html

Hope it helps.
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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: DS Questions - absolute integers - Help understand logic   [#permalink] 13 Sep 2010, 18:45
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