The Quest for 700: Weekly GMAT Challenge (Answer)

Yesterday, Manhattan GMAT posted a GMAT question on our blog. Today, they have followed up with the answer:

To attack this problem, we need to deal with two “strange symbol” formulas. We need to identify which of the five points given in the answer choices makes the two formulas equal.

One lengthy approach is to set the formulas equal to each other and perform algebra.
x # y = x ~ y
(x + y)3 = (xy)3
x3 + 3x2y + 3yx2 + y3 = x3 – 3x2y + 3yx2y3
(You don’t have to know that expansion – you can get it by FOILing.)
3x2y + y3 = –3x2yy3
6x2y + 2y3 = 0
y(6x2 + 2y2) = 0
Either y = 0 or 6x2 + 2y2 = 0. The latter is only true if both x and y equal 0. So we must have at least the condition that y = 0. Comparing to the choices, we should see that only (-2, 0) works.

Alternatively, by looking at the two formulas, we might search for a quick shortcut. When would these expressions be equal? By asking ourselves that question, we might realize that if y = 0, both formulas reduce to x3. So the two formulas will be equal whenever y = 0. The only viable option among the five answer choices is (-2, 0).

Finally, we can just “plug and chug” the answer choices. This might take more time, but as long as our substitutions and computations are correct, we will get the right answer.

The correct answer is D.

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