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What is the value of y – x^2 – x? (1) y = –3x (2) y =

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Source: — Data Sufficiency |

by GMATGuruNY » Tue Mar 14, 2017 5:43 pm
Anaira Mitch wrote:What is the value of y - x^2 - x?
(1) y = -3x

(2) y = -4(x + 1)
Statement 1: y = -3x
Case 1: x=0, with the result that y = -3(0) = 0
Here, y - x² - x = 0 - 0² - 0 = 0.
Case 2: x=1, with the result that y = -3(1) = -3
Here, y - x² - x = -3 - 1² - 1 = -5.
Since y - x² - x can be different values, INSUFFICIENT.

Statement 2: y = -4(x+1)
Case 3: x=0, with the result that y = -4(0+1) = -4
Here, y - x² - x = -4 - 0² - 0 = -4.
Case 4: x=1, with the result that y = -4(1+1) = -8
Here, y - x² - x = -8 - 1² - 1 = -10.
Since y - x² - x can be different values, INSUFFICIENT.

Statements combined:
Since we have two variables (x and y) and two distinct linear equations [ y=-3x and y=-4(x+1) ], we can solve for x and y.
Thus, the value of y - x² - x can be determined.
SUFFICIENT.

The correct answer is C.
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by Jay@ManhattanReview » Tue Mar 14, 2017 10:23 pm
Anaira Mitch wrote:What is the value of y - x^2 - x?

(1) y = -3x

(2) y = -4(x + 1)
Hi Anaira Mitch,

We have to find out the unique value of the expression y - x^2 - x.

Let's take each statement one by one.

S1: y = -3x

y - x^2 - x => -3x - x^2 - x => -(x^2 + 4x). We do not have the value x, thus the value of y - x^2 - x is indeterminable.

S2: y = -4(x + 1)

The same is the case with this one. We do not have the value x, thus the value of y - x^2 - x is indeterminable.

S1 and S2:

We have y = -3x and y = -4(x + 1)

Thus, -3x = -4(x + 1)

=> -3x = -4x -4 => x = -4 and y = 12.

=> y - x^2 - x = 12 - (-4)^2 - (-4) = 12 - 16 + 4 = 0. A unique value. Sufficient.

The correct answer: C

Hope this helps!

Relevant book: Manhattan Review GMAT Data Sufficiency Guide

-Jay
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