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Cube roots DS

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by righty123 » Wed Mar 10, 2010 12:24 pm
If x is not equal to y and if sqrtx = y , what is the value of y^3?

A) x = y ^ x
B) x ^ 3 = 8


OA D

I understand the logic behind the answers. However, I don't understand why the negative root is not considered when determining the sqrt of X. If the negative root is considered, a different value for Y would be the result.
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Source: — Data Sufficiency |
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by Testluv » Wed Mar 10, 2010 5:09 pm
I understand the logic behind the answers. However, I don't understand why the negative root is not considered when determining the sqrt of X. If the negative root is considered, a different value for Y would be the result.
Square roots are ALWAYS positive.

So, if x = rt9, then x = +3 but NOT -3.

But if x^2 = 9, then

x^2 - 9 = 0

which is a difference of squares:

x^2 - 3^2 = 0

(x-3)(x+3) = 0

x = +3 or -3.
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by girish3131 » Wed Mar 10, 2010 11:41 pm
wats the OA.... ???
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by papgust » Wed Mar 10, 2010 11:49 pm
Check the spoiler used by the thread starter
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by girish3131 » Wed Mar 10, 2010 11:57 pm
good one ...

Thanks!


1 small ques.... if x^2 = 4 then x = 2 and -2

BUT

when x^3 =8 then wat will be the values of x ..... ?
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by raisethebar » Thu Mar 11, 2010 1:32 am
is this soln correct?

sqrt (X) =y
squaring both side

X= Y^2 --1

1st stement is X=y^X --2

comparing 1 and 2 we can say X=2

sqrt(X) =Y
so cubing both the side

sqrt(2). sqrt(2). sqrt(2) =Y^3

so staement 1 is sufficient.

X^3=8
so x=2
so e know Y^3 as well.
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