Carcass wrote:
Which of the following expressions CANNOT be equal to 0 when \(x^2 − 2x = 3\)?
A. \(x^2 − 6x + 9\)
B. \(x^2 − 4x + 3\)
C. \(x^2 − x − 2\)
D. \(x^2 − 7x + 6\)
E. \(x^2 − 9\)
Kudos for the right answer and explanation
Question part of the project GRE Quantitative Reasoning Daily Challenge - (2021) EDITIONGRE - Math Bookx^2 − 2x = 3
Rearrange to get: x^2 − 2x - 3 = 0
Factor:
(x - 3)(x + 1) = 0
Conclusion: EITHER
(x - 3) = 0 OR
(x + 1) = 0
The correct answer will be the one in which we
CANNOT draw either of these conclusions.
IMPORTANT: To answer this question, we must start checking each answer choice. Given that most people start with A and work their way down, where is the sneakiest place to place the correct answer? Near the bottom. So, in these cases, I suggest that you start with E and work your way UP.
E. Can x^2 − 9 = 0?
Factor to get (x + 3)
(x - 3) = 0
So, (x + 3) = 0 or
(x - 3) = 0
ELIMINATE E
D. Can x^2 − 7x + 6 = 0
Factor to get (x - 1)(x - 6) = 0
So, (x - 1) = 0 or (x - 6) = 0
Since we
cannot conclude that
(x - 3) = 0 or
(x + 1) = 0, the correct answer must be D
Cheers,
Brent