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kaplan 49

Expert replies
by resilient » Thu Apr 03, 2008 10:51 pm
does root (x+12) = 2 + Root X

simplifed I squared the whole thing and the book only square the left side. Is that even possible. I thought what you do to one side you must do to another side.

1. (x-4)(x-10)=0
2.4(x+10)=5x +36


qa is b


stumped
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Source: — Data Sufficiency |

Re: kaplan 49

by lunarpower » Fri Apr 04, 2008 2:01 am
you should rephrase the statements before you do anything else, because they're both fantastically easy equations to solve. if you solve them, you get the following rephrases:
(1) x = 4 or 10
(2) x = 4


note that (2) is automatically SUFFICIENT, because it's one number; because it's one value, that number is guaranteed to give either a 'definitive yes' answer or a 'definitive no' answer.

to determine whether (1) is sufficient, just plug 4 and 10 into the equation and see what happens:
x = 4 --> rad(16) = 2 + rad(4) --> yes
x = 10 --> rad(22) = 2 + rad(10) --> no
INSUFFICIENT, because you have a 'yes' and a 'no'

note that there's no need to solve the equation, because the answer choices are so easy to rephrase!

--
Enginpasa1 wrote:does root (x+12) = 2 + Root X
I squared the whole thing and the book only square the left side. Is that even possible. I thought what you do to one side you must do to another side.
if you insist on solving the equation - which is, frankly, a waste of time, as you can just plug in the answer choices as is done above - then yes, you are absolutely right: you need to square both sides.

x + 12 = 4 + 4rad(x) + x
8 = 4rad(x)
2 = rad(x)
4 = x
so, the question rephrases to ... is x = 4?
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Re: kaplan 49

by codesnooker » Fri Apr 04, 2008 2:37 am
lunarpower wrote: so, the question rephrases to ... is x = 4?
Hello,
I am sorry for this but I am little bit disagreed with your solution. I have also thought the same solution but struck at the following point:-

Root (X) = ?
If X = 4, then Root(4) = +2 and -2.
So, in case if take it as -2 then the demanded condition would not be satisfied. So, in my opinion also the answer should be (E).

Since you a instructor, so of course you must know more than me. Let me know, in case of the real test, if any such condition arises then should we consider -negative value of root or not?

Thanks in Advance
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Re: kaplan 49

by Stuart@KaplanGMAT » Fri Apr 04, 2008 9:57 am
codesnooker wrote:
lunarpower wrote: so, the question rephrases to ... is x = 4?
Hello,
I am sorry for this but I am little bit disagreed with your solution. I have also thought the same solution but struck at the following point:-

Root (X) = ?
If X = 4, then Root(4) = +2 and -2.
So, in case if take it as -2 then the demanded condition would not be satisfied. So, in my opinion also the answer should be (E).

Since you a instructor, so of course you must know more than me. Let me know, in case of the real test, if any such condition arises then should we consider -negative value of root or not?

Thanks in Advance
By mathematical convention, the radical sign is translated as "the postive root of".

So, if you see a number or variable under a radical sign, you only have to worry about the positive root.

Understanding this issue has its biggest impact in data sufficiency. Let's look at a question that would test us on it:

What's the value of x?

(1) x = root(16)

(2) x^2 = 16

Statement (1) uses the radical sign, so we know that the only possible value of x is the positive root of 16, which is 4: sufficient.

Statement (2), on the hand, has two possible solutions. If x^2 = 16, then x could be positive 4 or negative 4: insufficient.

So, even though these statements seem to be providing the exact same information (if one were to make the mistake of squaring both sides of statement (1), one would actually get statement (2)), they're very different.
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ok

by resilient » Fri Apr 04, 2008 2:33 pm
I guess I am asking for the key to life. But how did you know to automatically backsolve. I have a bad habit of trying to plow through a question but if I just stepped back to analyze the complete situation, I would have had a nice answer.

I am learning the rephrasing is key! And on DS, success hinges on Rephrasing.
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