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# If x is a positive number and a=√x∗x−x,

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Intern
Joined: 17 Jun 2013
Posts: 24

Kudos [?]: 54 [0], given: 20

If x is a positive number and a=√x∗x−x, [#permalink]

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03 Feb 2014, 05:41
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Difficulty:

55% (hard)

Question Stats:

48% (00:43) correct 52% (01:05) wrong based on 152 sessions

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If x is a positive number and a=√x∗x−x, which of the following must be true?

I. a is even

II. a is positive

III. a is an integer

A)I only
B)II only
C)III only
D)I and II
E) None of the above
[Reveal] Spoiler: OA

Kudos [?]: 54 [0], given: 20

Math Expert
Joined: 02 Sep 2009
Posts: 41892

Kudos [?]: 129029 [2], given: 12187

Re: If x is a positive number and a=√x∗x−x, [#permalink]

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03 Feb 2014, 05:50
2
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Expert's post
gmatgambler wrote:
If x is a positive number and a=√x∗x−x, which of the following must be true?

I. a is even

II. a is positive

III. a is an integer

A)I only
B)II only
C)III only
D)I and II
E) None of the above

If $$x$$ is a positive number and $$a=\sqrt{x} * x - x$$, which of the following must be true?

I. $$a$$ is even
II. $$a$$ is positive
III. $$a$$ is an integer

A. I only
B. II only
C. III only
D. I and II
E. None of the above

Note that we are asked "which of the following MUST be true, not COULD be true. For such kind of questions if you can prove that a statement is NOT true for one particular set of numbers, it will mean that this statement is not always true and hence not a correct answer.

If $$x=\frac{1}{4}$$ then $$a=\sqrt{x} * x - x=\frac{1}{2}*\frac{1}{4}-\frac{1}{4}=-\frac{1}{8}$$. Now, $$-\frac{1}{8}$$ is not an integer at all (hence not even) and also not positive, so none of the options MUST be true.

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Kudos [?]: 129029 [2], given: 12187

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Re: If x is a positive number and a=√x∗x−x, [#permalink]

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03 Feb 2014, 22:44
1
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Expert's post
1
This post was
BOOKMARKED
gmatgambler wrote:
If x is a positive number and a=√x∗x−x, which of the following must be true?

I. a is even

II. a is positive

III. a is an integer

A)I only
B)II only
C)III only
D)I and II
E) None of the above

Also, let me point out how we arrive at the numbers that we should try here. Given x is a positive number and $$a =\sqrt{x}*x - x$$.
$$\sqrt{x}$$ makes us think of perfect squares. We try to put in a perfect square such as 4 and see that all conditions are met.
So now we need to think of the cases when the condition may not be met.

Let's figure out whether a must be positive.
$$\sqrt{x}$$ is positive and x is positive but for a to be positive, $$\sqrt{x}*x$$ MUST be greater than x. Will $$\sqrt{x}*x$$ be less than x in any case? It will be if $$\sqrt{x}$$ is less than 1 i.e. a fraction. So this leads us to x = 1/4. We must try this value. When x = 1/4, we see that a is not positive, is not an integer and hence the question of even or odd doesn't arise.

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Karishma
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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Kudos [?]: 17369 [1], given: 232 Manager Status: Kitchener Joined: 03 Oct 2013 Posts: 96 Kudos [?]: 26 [0], given: 144 Location: Canada Concentration: Finance, Finance GPA: 2.9 WE: Education (Education) If x is a positive number and a=√x∗x−x, [#permalink] ### Show Tags 18 Jan 2015, 14:22 Bunuel wrote: gmatgambler wrote: If x is a positive number and a=√x∗x−x, which of the following must be true? I. a is even II. a is positive III. a is an integer A)I only B)II only C)III only D)I and II E) None of the above If $$x$$ is a positive number and $$a=\sqrt{x} * x - x$$, which of the following must be true? I. $$a$$ is even II. $$a$$ is positive III. $$a$$ is an integer A. I only B. II only C. III only D. I and II E. None of the above Note that we are asked "which of the following MUST be true, not COULD be true. For such kind of questions if you can prove that a statement is NOT true for one particular set of numbers, it will mean that this statement is not always true and hence not a correct answer. If $$x=\frac{1}{4}$$ then $$a=\sqrt{x} * x - x=\frac{1}{2}*\frac{1}{4}-\frac{1}{4}=-\frac{1}{8}$$. Now, $$-\frac{1}{8}$$ is not an integer at all (hence not even) and also not positive, so none of the options MUST be true. Answer: E. [b][color=#0d004c]I did as Bunuel did where I used 1/4 as option and I subtrac 1/4 from 1/4 and then Multiple by sqrt 1/4 the result was always zero. So my answer was E because 0 is both even and integer abd there is no option include both A and C. Does my reason to chosen was true? _________________ Click +1 Kudos if my post helped Kudos [?]: 26 [0], given: 144 EMPOWERgmat Instructor Status: GMAT Assassin/Co-Founder Affiliations: EMPOWERgmat Joined: 19 Dec 2014 Posts: 9987 Kudos [?]: 3412 [0], given: 172 Location: United States (CA) GMAT 1: 800 Q51 V49 GRE 1: 340 Q170 V170 Re: If x is a positive number and a=√x∗x−x, [#permalink] ### Show Tags 18 Jan 2015, 19:22 HI All, The "font" (and lack of parentheses) makes this question a little tough to interpret. I believe that the intent is the following: If X is POSITIVE, and A = (\sqrt{X})(X) - X which of the following MUST be true? Since the question asks what MUST be true, we can quickly eliminate possibilities by proving what is NOT always true (by TESTing VALUES) IF.... A = 1, then (\sqrt{1})(1) - 1 = (1)(1) - 1 = 0 This result is NOT positive. Eliminate Roman Numeral II and Answers B and D. IF.... A = 2, then (\sqrt{2})(2) - 2 = NOT an integer. Eliminate Roman Numerals I and III and Answers A and C. Final Answer: [Reveal] Spoiler: E GMAT assassins aren't born, they're made, Rich _________________ 760+: Learn What GMAT Assassins Do to Score at the Highest Levels Contact Rich at: Rich.C@empowergmat.com # Rich Cohen Co-Founder & GMAT Assassin Special Offer: Save$75 + GMAT Club Tests Free
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Kudos [?]: 3412 [0], given: 172

Veritas Prep GMAT Instructor
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Re: If x is a positive number and a=√x∗x−x, [#permalink]

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18 Jan 2015, 21:33
1
KUDOS
Expert's post
23a2012 wrote:
Bunuel wrote:
gmatgambler wrote:
If x is a positive number and a=√x∗x−x, which of the following must be true?

I. a is even

II. a is positive

III. a is an integer

A)I only
B)II only
C)III only
D)I and II
E) None of the above

If $$x$$ is a positive number and $$a=\sqrt{x} * x - x$$, which of the following must be true?

I. $$a$$ is even
II. $$a$$ is positive
III. $$a$$ is an integer

A. I only
B. II only
C. III only
D. I and II
E. None of the above

Note that we are asked "which of the following MUST be true, not COULD be true. For such kind of questions if you can prove that a statement is NOT true for one particular set of numbers, it will mean that this statement is not always true and hence not a correct answer.

If $$x=\frac{1}{4}$$ then $$a=\sqrt{x} * x - x=\frac{1}{2}*\frac{1}{4}-\frac{1}{4}=-\frac{1}{8}$$. Now, $$-\frac{1}{8}$$ is not an integer at all (hence not even) and also not positive, so none of the options MUST be true.

[b][color=#0d004c]I did as Bunuel did where I used 1/4 as option and I subtrac 1/4 from 1/4 and then Multiple by sqrt 1/4

the result was always zero. So my answer was E because 0 is both even and integer abd there is no option include both A and C. Does my reason to chosen was true?

You cannot subtract x first and then multiply by root(x). The multiplication has to be done first because sequence of operators follows the rule of PEMDAS/BODMAS. I suggest you to check this out from a Math book or online.
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Re: If x is a positive number and a=√x∗x−x, [#permalink]

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30 Oct 2016, 08:22
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Re: If x is a positive number and a=√x∗x−x,   [#permalink] 30 Oct 2016, 08:22
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