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

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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.

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If x is an integer, is x|x| < 2^x? [#permalink]
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If x is an integer, is x|x| < 2^x?

(1) x < 0
(2) x = -10

I can understand the second part:
-10|-10| < 2^-10 --> -10 * 10 < 1/2 ^ 10
|-10| --> reduced to 10 as its numeric.. is my reasoning correct?
B is sufficient

For (1) .. however i am not able to decipher anything..
-x|-x| < 2^-x --> -x * -x < 1/2 ^x
|-x| --> reduced to -x as x < 0 .. is my reasoning correct?
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Thanks Kp.

But if x<0 so we get |-x| => -x
Am i missing something?
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DS Questions - absolute integers - Help understand logic [#permalink]
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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]
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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.

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]
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]
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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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Re: If x is an integer, is x|x|<2^x ? [#permalink]
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reza52520 wrote:
If x is an integer, is x|x|<2^x ?

(1) x < 0
(2) x = -10

Question : Is x|x|<2^x ?

Statement 1: x < 0

For x to be Negative LHS i.e. x|x| will always be NEGATIVE
and 2^x will be positive for any value of x
i.e. x|x|<2^x will always be true
SUFFICIENT

Statement 1: x = -10
For x =-10 LHS i.e. x|x| will always be NEGATIVE (-100)
and 2^x will be positive for given x (1/2^10)
i.e. x|x|<2^x will always be true
SUFFICIENT

Originally posted by GMATinsight on 27 Jul 2015, 06:43.
Last edited by GMATinsight on 27 Jul 2015, 06:44, edited 1 time in total.
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Re: If x is an integer, is x|x| < 2^x? [#permalink]
cucrose wrote:
If x is an integer, is x|x| < 2^x?

(1) x < 0
(2) x = -10

Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

There is 1 variable. Thus D is the answer most likely.

Condition 1) $$x < 0$$
Since $$|x| = -x$$ if $$x < 0$$, the question $$x|x| < 2^x$$ is equivalent to $$-x^2 < 2^x$$.
We have the left hand side $$-x^2 < 0$$ and the right hand side $$2^x > 0$$ all times.
Thus $$-x^2 < 0 < 2^x$$.
This is sufficient.

Condition 2) $$x = -10$$
Since $$x = -10$$ is negative, by the same logic of the condition 1), this condition is also sufficient.

Therefore, the answer is D as expected.

-> For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.
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If x is an integer, is x|x|<2^x ?

(1) x < 0
(2) x = -10

Target question: Is x|x|< 2^x ?

Given: x is an integer

Statement 1: x < 0
In other words, x is NEGATIVE
So, x|x| = (NEGATIVE)(|NEGATIVE|) = (NEGATIVE)(POSITIVE) = NEGATIVE

IMPORTANT: 2^x will be POSITIVE for all values of x.

Since x|x| must be NEGATIVE, and since 2^x must be POSITIVE, we can be certain that x|x|< 2^x
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: x = -10
So, x|x| = (-10)(|-10|) = (-10)(10) = -100 = a NEGATIVE
On the other hand, 2^x = 2^(-10) = 1/(2^10) = some POSITIVE number
Since x|x| is NEGATIVE, and since 2^x must be POSITIVE, we can be certain that x|x|< 2^x
Since we can answer the target question with certainty, statement 2 is SUFFICIENT

Originally posted by BrentGMATPrepNow on 23 Aug 2017, 13:52.
Last edited by BrentGMATPrepNow on 12 Nov 2019, 18:12, edited 1 time in total.
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Re: If x is an integer, is x|x|<2^x ? [#permalink]
Hi,

For statement 1 I tested values i.e. x = -1 or x = -2.This is more so a question regarding reciprocals and inequalities. If x = -2, then -2 (|-2|) = 2^-2. Then, this is equal to -2 (2) = 1/2^2. In the second step where I converted 2^-2 to 1/2^2 -- would I have to also flip the other side to become 1 / -2 (2) or is that wrong?

Thanks,

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Re: If x is an integer, is x|x|<2^x ? [#permalink]
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infinitemac wrote:
Hi,

For statement 1 I tested values i.e. x = -1 or x = -2.This is more so a question regarding reciprocals and inequalities. If x = -2, then -2 (|-2|) = 2^-2. Then, this is equal to -2 (2) = 1/2^2. In the second step where I converted 2^-2 to 1/2^2 -- would I have to also flip the other side to become 1 / -2 (2) or is that wrong?

Thanks,

infinitemac

No. The right hand side is $$2^{(-2)}$$, which is the same as $$\frac{1}{2^2}$$ but the left hand side (-2*|-2|) stays the same.

Negative powers:
$$a^{-n}=\frac{1}{a^n}$$
Important: you cannot rise 0 to a negative power because you get division by 0, which is NOT allowed. For example, $$0^{-1} = \frac{1}{0}=undefined$$.

8. Exponents and Roots of Numbers

Check below for more:
ALL YOU NEED FOR QUANT ! ! !

Hope it helps.
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Re: If x is an integer, is x|x|<2^x ? [#permalink]
If x is an integer, is x|x|<2^x ?

(1) x < 0
(2) x = -10

Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

Since we have 1 variables and 0 equation, D could be the answer most likely.

Condition 1)
Since x < 0 and |x|≥0, x|x|≤0.
2^x > 0
Thus x|x| < 2^x.
This is sufficient.

Condition 2)
Since x = -10, x|x| = (-10)*10 = -100 < 0
And 2^(-10) = 1/(2^10) > 0
Thus x|x| < 2^x
This is also sufficient.

For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both con 1) and con 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. D is most likely to be the answer using con 1) and con 2) separately according to DS definition. Obviously, there may be cases where the answer is A, B, C or E.
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Re: If x is an integer, is x|x|<2^x ? [#permalink]
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Bunuel wrote:
If x is an integer, is x|x|<2^x ?

(1) x < 0
(2) x= -10

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

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 $$x*|x|<0<2^x$$. Sufficient.

(2) x = -10. The same here $$x*|x|=-100<0<\frac{1}{2^{10}}$$. Sufficient.

what is wrong in my approach :

x |x| < 2^x
x *sqrt(x^2) < 2^x
square on both sides,
x^2 * x^2 < 2^2x
x^4 < 2^2x

given 1 stmt, x as -ve, always x^4 > 2^2x, whereas I know i am making some mistake.
are we not allowed to take square on both sides?
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Re: If x is an integer, is x|x|<2^x ? [#permalink]
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Avinash_R1 wrote:
Bunuel wrote:
If x is an integer, is x|x|<2^x ?

(1) x < 0
(2) x= -10

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

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 $$x*|x|<0<2^x$$. Sufficient.

(2) x = -10. The same here $$x*|x|=-100<0<\frac{1}{2^{10}}$$. Sufficient.

what is wrong in my approach :

x |x| < 2^x
x *sqrt(x^2) < 2^x
square on both sides,
x^2 * x^2 < 2^2x
x^4 < 2^2x

given 1 stmt, x as -ve, always x^4 > 2^2x, whereas I know i am making some mistake.
are we not allowed to take square on both sides?

We can raise both parts of an inequality to an even power if we know that both parts of an inequality are non-negative (the same for taking an even root of both sides of an inequality). Here x|x| is negative if x is negative, so we cannot square.

RAISING INEQUALITIES TO EVEN/ODD POWER

1. We can raise both parts of an inequality to an even power if we know that both parts of an inequality are non-negative (the same for taking an even root of both sides of an inequality).
For example:
$$2<4$$ --> we can square both sides and write: $$2^2<4^2$$;
$$0\leq{x}<{y}$$ --> we can square both sides and write: $$x^2<y^2$$;

But if either of side is negative then raising to even power doesn't always work.
For example: $$1>-2$$ if we square we'll get $$1>4$$ which is not right. So if given that $$x>y$$ then we cannot square both sides and write $$x^2>y^2$$ if we are not certain that both $$x$$ and $$y$$ are non-negative.

2. We can always raise both parts of an inequality to an odd power (the same for taking an odd root of both sides of an inequality).
For example:
$$-2<-1$$ --> we can raise both sides to third power and write: $$-2^3=-8<-1=-1^3$$ or $$-5<1$$ --> $$-5^3=-125<1=1^3$$;
$$x<y$$ --> we can raise both sides to third power and write: $$x^3<y^3$$.

Adding, subtracting, squaring etc.: Manipulating Inequalities.

9. Inequalities

For more check Ultimate GMAT Quantitative Megathread

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Re: If x is an integer, is x|x|<2^x ? [#permalink]
Hi All,

We're told that X is an integer. We're asked if X|X| < 2^X. This is a YES/NO question. We can answer it with a bit of Number Property knowledge.

1) X < 0

With Fact 1, we know that X is NEGATIVE. By definition, that means...
X|X| = (Neg)|Neg| = Negative
2^(Negative) = Positive
Thus, X|X| will ALWAYS be less than 2^X and the answer to the question is ALWAYS YES.
Fact 1 is SUFFICIENT

2) X = -10

With the value of X, we can absolutely answer the question (we would just need to plug in that value:
Is (-10)|-10| < 2^(-10)?
The answer to the question IS yes, but we don't have to actually do that work. There would be just one answer to the question, so it doesn't really matter what that one answer is.
Fact 2 is SUFFICIENT

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Re: If x is an integer, is x|x| < 2^x? [#permalink]
Hi All,

We're told that X is an INTEGER. We're asked if X|X| is less than 2^X. This is a YES/NO question. This DS question is built around a couple of Number Properties, so you can actually solve it without doing much math.

1) X < 0

This Fact tells us that X is NEGATIVE, which means....

X|X| = (negative)|negative| = (-)(+) = ALWAYS negative
2^X = 2^(negative) = 1/[2^(positive)] = ALWAYS positive

Thus, the answer to the question is ALWAYS YES.
Fact 1 is SUFFICIENT

2) X = -10

Since this tells us the only value of X, there will be only one answer to the given question (the answer happens to be YES).
Fact 2 is SUFFICIENT