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If x<0, then root(-x*|x|) is: [#permalink]
 29 Jul 2009, 16:42
Question Stats:
50% (01:24) correct
50% (00:26) wrong based on 20 sessions
If x<0, then \sqrt{-x|x|} is: A. -x B. -1 C. 1 D. x E. \sqrt{x}
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It is actually A.
suppose x is -2 then you have sqrt(2*2) = sqrt(4) = 2 = -x
note that x < 0 as otherwise the function does not exist.
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nss123 wrote: I'm having trouble understanding how they get to the answer, though it should be very simple:
If x<0, then \sqrt{-x|x|} is:
A) -x
B) -1
C) 1
D) x
E) \sqrt{x}
I got D, b/c I'm thinking, if x is negative, then taking the opposite of that will be positive, mulitiplied by the absolute value of x, which is also positive. That gives you x^2, which you take the square root of, to get x
Any help would be great. are you sure the question is correct . I remember seeing the question as sqrt(-x/|x|) In that case ans is -1. but the problem you have stated, the ans should be x.
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hailtothethief23 wrote: It is actually A.
suppose x is -2 then you have sqrt(2*2) = sqrt(4) = 2 = -x
note that x < 0 as otherwise the function does not exist. I had same question today \sqrt{4} = + or - 2. So answer should be + or - x. I don't get how can OA be -x
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saxenashobhit wrote: hailtothethief23 wrote: It is actually A.
suppose x is -2 then you have sqrt(2*2) = sqrt(4) = 2 = -x
note that x < 0 as otherwise the function does not exist. I had same question today \sqrt{4} = + or - 2. So answer should be + or - x. I don't get how can OA be -x GMAT is dealing only with Real Numbers: Integers, Fractions and Irrational Numbers. 2. Any nonnegative real number has a unique non-negative square root called the principal square root and unless otherwise specified, the square root is generally taken to mean the principal square root. When the GMAT provides the square root sign for an even root, such as \sqrt{x} or \sqrt[4]{x}, then the only accepted answer is the positive root. That is, \sqrt{25}=5, NOT +5 or -5. In contrast, the equation x^2=25 has TWO solutions, +5 and -5. Even roots have only non-negative value on the GMAT.Odd roots will have the same sign as the base of the root. For example, \sqrt[3]{125} =5 and \sqrt[3]{-64} =-4. 3. \sqrt{x^2}=|x|. The point here is that as square root function can not give negative result then \sqrt{some \ expression}\geq{0}. So \sqrt{x^2}\geq{0}. But what does \sqrt{x^2} equal to? Let's consider following examples: If x=5 --> \sqrt{x^2}=\sqrt{25}=5=x=positive; If x=-5 --> \sqrt{x^2}=\sqrt{25}=5=-x=positive. So we got that: \sqrt{x^2}=x, if x\geq{0}; \sqrt{x^2}=-x, if x<0. What function does exactly the same thing? The absolute value function: |x|=x, if x\geq{0} and |x|=-x, if x<0. That is why \sqrt{x^2}=|x|. BACK TO THE ORIGINAL QUESTION:1. If x<0, then \sqrt{-x*|x|} equals:A. -xB. -1C. 1D. xE. \sqrt{x}\sqrt{-x*|x|}=\sqrt{(-x)*(-x)}=\sqrt{x^2}=|x|=-x. Note that as x<0 then -x=-negative=positive, so \sqrt{-x*|x|}=-x=positive as it should be. Or just substitute the some negative x, let x=-5<0 --> \sqrt{-x*|x|}=\sqrt{-(-5)*|-5|}=\sqrt{25}=5=-(-5)=-x. Answer: A. Hope it's clear.
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I got that wrong, thanks "Bunuel"
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Thanks. Only catch which I wasn't aware was that on square roots I should consider positive value alone. Although this is wrong based on general Math
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Thank you for this post.
Last edited by mbaiseasy on 06 Dec 2012, 05:57, edited 1 time in total.
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nss123 wrote: I'm having trouble understanding how they get to the answer, though it should be very simple:
If x<0, then \sqrt{-x|x|} is:
A) -x
B) -1
C) 1
D) x
E) \sqrt{x}
I got D, b/c I'm thinking, if x is negative, then taking the opposite of that will be positive, mulitiplied by the absolute value of x, which is also positive. That gives you x^2, which you take the square root of, to get x.
Any help would be great. Square root of any number CANNOT be negative... The answer is quite apparent and should not take more than 10 seconds to figure out.. B & D can be immediately eliminated.. There is no evidence to show C.(C takes a constant value for the root of a variable) and E is an imaginary number... A is clearly the right answer..
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Re: If x<0, then root(-x*|x|) is: [#permalink]
06 Dec 2012, 05:56
Let x=-5
\sqrt{-(-5)|-5|}=\sqrt{25}=5
5 = -(-5) = -x
Answer: -x or A
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Re: If x<0, then root(-x*|x|) is:
[#permalink]
06 Dec 2012, 05:56
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