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x^3-y^3=?

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by Max@Math Revolution » Wed Feb 13, 2019 12:32 am

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Answers

A

B

C

D

E

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Difficulty—

[GMAT math practice question]

x^3-y^3=?

1) x-y=0
2) |x|=|y|
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Source: — Data Sufficiency |
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by Max@Math Revolution » Fri Feb 15, 2019 12:00 am
=>

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.

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

x^3-y^3 = (x-y)(x^2+xy+y^2)

Condition 1)
If x - y = 0, then x^3-y^3 = (x-y)(x^2+xy+y^2) = 0.
Thus, condition 1) is sufficient.

Condition 2)
If x = 1 and y = 1, then x^3-y^3 = 1 - 1 = 0.
If x = 1 and y = -1, then x^3-y^3 = 1 -(-1) = 2.
Condition 2) is not sufficient since it does not yield a unique solution.

Therefore, A is the answer.
Answer: A
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