What is a^3 - b^3?

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What is a^3 - b^3?

by Max@Math Revolution » Fri Feb 21, 2020 12:40 am
[GMAT math practice question]

What is a^3 - b^3?

1) 1/a-1/b=2
2) 1/a^2+1/b^2=3

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Re: What is a^3 - b^3?

by Max@Math Revolution » Sun Feb 23, 2020 6:10 pm
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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.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have 2 variables (a and b) and 0 equations, C is most likely 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)

Since we have 1/a – 1/b = 2
b/ab – a/ab = 2 (common denominator)
(b – a)/ab = 2 (subtracting fractions
a – b = -2ab (multiplying both sides by ab

Since we have 1/a^2 + 1/b^2 = 3
b^2/a^2b^2 + a^2/a^2b^2 = 3 (common denominator)
(b^2 + a^2)/a^2b^2 = 3 (adding fractions)
a^2 + b^2 = 3a^2b^2 (multiplying both sides by a^2b^2)

Then we have a^2 + b^2 = (a-b)^2 + 2ab
a^2 + b^2 = (-2ab)^2 + 2ab (substituting -2ab in for a – b)

a^2 + b^2 = 3a^2b^2 or a^2b^2 + 2ab = 0.
We have ab = -2 and a – b= -2ab = 4 from ab(ab+2)=0, since ab ≠ 0.
a^3-b^3 = (a-b)^3 + 3ab(a-b) = 4^3 + 3(-2)4 = 64 – 24 = 40.

Since both conditions together yield a unique solution, they are sufficient.

Therefore, C is the answer.
Answer: C

Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B, or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D, or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.