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Square Root problem, take a look

Expert replies
by sreecharan » Mon Sep 14, 2009 11:27 am
Is sqrt(x) a prime number?

(1)|3x-7| = 2x+2

(2) x^2 = 9x



answer below




From statement one, we know that x must be 1 or 9. From statement two, we know that x must be 0 or 9. Thus, since both statements must be true, we can deduce that x must be 9. Therefore sqrt x = 3.


I dont get this, if x = 9 then sqrt x can be +3 or -3, so how is the answer C, I think it should be E.
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Source: — Data Sufficiency |

by grockit_jake » Mon Sep 14, 2009 6:00 pm
Techincally when you have an equation x^2 = 9, then x = +/-3.

For some reason, when you have rt9, the answer is only +3. There is some complex derivation for this that everyone forgets in 9th grade.

BUT the question is asking if sqrtx is prime. 3 is prime, so the answer is Yes. (C).

Negative numbers are technically not considered in prime question. Wording was confusing though, not the best Q.
Jake Becker
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Grockit Test Prep
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