Data Sufficiency

[GMAT math practice question]

Is x + y < -1?

1) |x| > x and |y| > y
2) x^2+y^2 > 1

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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.

Since we have 2 variables (x and y) and 0 equations, C is most likely to be the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)
Condition 1) tells us that x and y are negative numbers.
By condition 2), (x+y)^2 = x^2 + 2xy +y^2 > x^2 + y^2 > 1 and (x+y)^2 > 1 since x and y are negative and xy > 0.

Since x + y < 0, we must have x + y < -1.
Conditions 1) & 2) are sufficient, when considered together.

Therefore, C is the answer.
Answer: C