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What is the value of a?

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What is the value of a?

by Max@Math Revolution » Thu Dec 05, 2019 11:58 pm
[GMAT math practice question]

What is the value of a?

1) |x - 1| = -(ax - 1)^2.
2) |ax - 1| = -√(x - 1).
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Source: — Data Sufficiency |
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by Max@Math Revolution » Thu Dec 05, 2019 11:59 pm
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by Max@Math Revolution » Sun Dec 08, 2019 5:15 pm
=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

Condition 1)
|x - 1| = -(ax - 1)^2 is equivalent to x = 1 and a = 1 for the following reason.
|x - 1| = -(ax - 1)^2
=> |x - 1| + (ax - 1)^2 = 0
=> x - 1 = 0 and ax - 1 = 0
=> x - 1 = 0 and a - 1 = 0
=> x = 1 and a = 1

Since condition 1) yields a unique solution, it is sufficient.

Condition 2)
|ax - 1| = -√(x - 1) is equivalent to x = 1 and a = 1 for the following reason.
|ax - 1| = -√(x - 1)
=> |ax - 1| + √(x - 1) = 0
=> ax - 1 = 0 and √(x-1) = 0
=> a - 1 = 0 and x - 1 = 0
=> a = 1 and x = 1

Since condition 2) yields a unique solution, it is sufficient.

Therefore, D is the answer.
Answer: D

Note: Tip 1) of the VA method states that D is most likely the answer if condition 1) gives the same information as condition 2).
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