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Algebra - Quadratic Example

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Algebra - Quadratic Example

by diegocml » Thu Apr 07, 2016 8:41 am
Hello guys,

I was wondering if someone could tell me why exactly fact 1 in this DS question is sufficient.

When I came across it, I thought that replacing "x(squared) + 6x" in the question statement to '40' seemed to obvious. What if the question statement stated "x(squared) + 6x = -9" I basically didn't do this move (subtracted 9 from both sides) because I thought it was too easy/good to be true.

What's the value of x(squared) + 6x + 9 ?

Fact 1) x(squared) + 6x = 40
Fact 2) x + 3 = 7

I hope I made my question clear.

Thanks a lot for your help and time.
Cheers,
Diego
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1st GMAT attemp: 410 (Q18 V27)
2nd GMAT attemp: 490 (Q35 V23)
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Source: — Data Sufficiency |
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by MartyMurray » Thu Apr 07, 2016 1:04 pm
diegocml wrote:What's the value of x² + 6x + 9?

(1) x² + 6x = 40

(2) x + 3 = 7
Statement 1:

Given that x² + 6x = 40, we can substitute 40 into the expression from the question.

40 + 9 = 49

Sufficient.

Statement 2:

x² + 6x + 9 = (x + 3)²

Since statement 2 provides the value of x + 3, we can calculate the value of x² + 6x + 9.

Sufficient.

The correct answer is D.
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by [email protected] » Thu Apr 07, 2016 5:37 pm
Hi Diego,

Certain DS questions really are that simple; one (or both) of the Facts can simply be 'substituted in' to the prompt to answer the given question. Remember that if you're going to define a given Fact as insufficient, then you need proof that there's more than one answer to the given question. If you can't prove that there's more than one answer, then by default you would have to assume that it was sufficient.

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