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Query on data sufficeincy! Help!!

Expert replies
by [email protected] » Mon May 04, 2009 6:21 am
Hi all,

I have a query on the following question

Q) Is x^5 + 4x^3 + 8x^2 + 32 = 128

1) x^2 - 5x + 6 = 0
2) x^2 +5x +6 = 0

My query is:
When you solve the above equation with help of Statement 1 ie x = 2 or 3, both values satisfy the equation.
When you solve it with help of Statement 2, ie x = -2, -3, it does not satisfy the equation.

In such situations, what should be the answer..... I answered as option D ie either statement is suffecient to answer (either in negative or affirmative) if the equation equals 128.
However the answer in the mock test was A i.e. Statement 1 alone is suffiecinet???

Can some one please clarify if I am wrong and missing some important point?

Sagar
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Source: — Data Sufficiency |

Re: Query on data sufficeincy! Help!!

by Ian Stewart » Mon May 04, 2009 7:12 am
[email protected] wrote: My query is:
When you solve the above equation with help of Statement 1 ie x = 2 or 3, both values satisfy the equation.
When you solve it with help of Statement 2, ie x = -2, -3, it does not satisfy the equation.

In such situations, what should be the answer..... I answered as option D ie either statement is suffecient to answer (either in negative or affirmative) if the equation equals 128.
However the answer in the mock test was A i.e. Statement 1 alone is suffiecinet???
A few issues here. From S1, you can certainly conclude that either x = 2 or x = 3. Plugging x = 2 into the equation in the question, we do find that the equation might be true. On the other hand, you can quickly see that x = 3 will not work in the equation:

x^5 + 4x^3 + 8x^2 + 32 = 128

Since 3^5 is already larger than 128, and the other terms on the left side of the equation are all positive, there's no way x = 3 can be a solution here. Or you could notice that if x is odd, the left side of the equation is odd (it's the sum of one odd number, x^5, and three even numbers), so can't possibly be equal to 128, an even number. So S1 is not sufficient, and neither A nor D can be correct here.

Then there's Statement 2. The second statement clearly contradicts the first; they have no solutions in common. This can *never* happen in a real GMAT question, since if it did, it would make no logical sense to consider both statements together. Of course, taken alone, S2 is sufficient to give a 'no' answer to the question, but the question is very poorly designed, and if the OA provided is A, something is very wrong with the source. Where is the question from?
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com
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Data Sufficiency - Thanks

by [email protected] » Mon May 04, 2009 6:58 pm
Thanks Ian for your help. Its from a mock test.

Sagar
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