Data Sufficiency

[GMAT math practice question] 5.31

If f(x) = x^2 - x + 1, is f(p)>f(q)?

1) p<q
2) p^2>q^2

=>

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.
Modifying the question:
f(p) > f(q)
=> f(p) - f(q) > 0
=> ( p^2 - p + 1 ) - ( q^2 - q + 1 ) > 0
=> ( p^2 - q^2 ) - ( p - q ) > 0
=> ( p + q )( p - q ) - ( p - q ) > 0
=> ( p + q - 1 )( p - q ) > 0
Since we have 2 variables (p and q) 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 2) tells us that p^2 - q^2 = (p + q)(p - q) > 0. Since p - q < 0 by condition 1), we must have p + q < 0. It follows that p + q - 1 < 0, and
f(p) - f(q) = (p + q - 1)(p - q) > 0.
So, f(p) > f(q), and both conditions are sufficient, when taken together.
Therefore, C is the answer.