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Root of the Matter

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Milanproda
 Post subject: Root of the Matter
 Post Posted: Thu Jul 21, 2011 11:11 pm 
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Course Students


Posts: 48
What is the value of x?

(1) sqrt4x= 6

(2) x2 = 81






The OA is A.

My question is this..if you square a square root, don't you get the absolute value of that root. In this example, if you square out the 4 and divide over to the side, you get sqroot of X= 3. I thought that when you square a number under the variable sign that you get the absolute value of the that number (in this case 3 or -3)?
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george.kourdin
 Post subject: Re: Root of the Matter
 Post Posted: Fri Jul 22, 2011 1:32 pm 
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Course Students


Posts: 98
think you are getting confused with the order of operations

yes its technically putting the number in absolute value brackets but that number needs to be squared. the way you solved the problem is fine, however, once we get to sqrt(x) = 3 we cant just be like...cool so abs(x) = 3 therefore x = +/- 3. you forgot to square both sides and since you can not take a square root of a negative number on the gmat x will be 9.

imho an easier way is to square both sides right away.
sqrt (4x) = 6
4x = 36
x = 9

the way you got to sqrt (x) = 3 is perfectly fine but its seems unnecessarily complicated, especially if you are going to end up squaring both sides at the end anyways
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Milanproda
 Post subject: Re: Root of the Matter
 Post Posted: Fri Jul 22, 2011 8:19 pm 
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Course Students


Posts: 48
Yes I am definitely missing something here, I was always under the impression that squaring a root meant having to find the absolute value.

Thx
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jnelson0612
 Post subject: Re: Root of the Matter
 Post Posted: Sat Jul 30, 2011 9:55 pm 
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ManhattanGMAT Staff


Posts: 1857
Milan, I'll assume you are okay unless you let us know otherwise. Thanks!

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Jamie Nelson
ManhattanGMAT Instructor
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