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14 Nov 2014, 09:53
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Difficulty:
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Question Stats:
65% (01:59) correct
35%
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wrong
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What is the value of \(\frac{1}{x^2 - y^2} - \frac{1}{x^2 + 2xy + y^2}\)?
(1) \(2y = x^2 - y^2\)
(2) \(x + y = 4\)
(1) \(2y = x^2 - y^2\)
(2) \(x + y = 4\)
17 Nov 2014, 12:13
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Bunuel
Official Solution:
What is the value of \(\frac{1}{x^2 - y^2} - \frac{1}{x^2 + 2xy + y^2}\)?
We must determine the value of the expression \(\frac{1}{x^2 - y^2} - \frac{1}{x^2 + 2xy + y^2}\). When a problem presents an opportunity to factor or find a common denominator, it is usually a good idea to do so.
First, factor the denominator of each fraction. The denominator of the fraction on the left is a difference of squares: \(\frac{1}{x^2 - y^2} = \frac{1}{(x + y)(x - y)}\). The denominator of the fraction on the right is the expanded form of the quadratic expression \((x + y)^{2}\): \(\frac{1}{x^2 + 2xy + y^2} = \frac{1}{(x + y)^2}\).
Thus, the fraction can be rewritten: \(\frac{1}{(x + y)(x - y)} - \frac{1}{(x + y)^2}\). The common denominator of these two fractions is \((x + y)^{2}(x - y)\). Multiply the first fraction by \(\frac{x + y}{x + y}\) and the second fraction by \(\frac{x - y}{x - y}\) and rewrite: \(\frac{x + y}{(x + y)^{2}(x - y)} - \frac{x - y}{(x + y)^{2}(x - y)}\).
Combine the terms by subtracting: \(\frac{x + y - (x - y)}{(x + y)^{2}(x - y)} = \frac{2y}{(x + y)^{2}(x - y)}\).
Statement 1 says that \(2y = x^2 - y^2\), or \(2y = (x + y)(x - y)\). Substitute this into the fraction that we derived above: \(\frac{2y}{(x + y)^{2}(x - y)} = \frac{(x + y)(x - y)}{(x + y)^{2}(x - y)}\). Cancel the factors that appear in both the numerator and the denominator, leaving \(\frac{1}{x + y}\). Without more information about \(x\) or \(y\), we cannot determine the value of this fraction. Statement 1 is NOT sufficient. Eliminate answer choices A and D. The correct answer choice is B, C, or E.
Statement 2 says that \(x + y = 4\). In this case, it will be easier to substitute into the expression \(\frac{1}{(x + y)(x - y)} - \frac{1}{(x + y)^2}\). Doing so gives: \(\frac{1}{4(x - y)} - \frac{1}{(4)^2}\). Without more information about \(x\) and \(y\), however, we cannot determine the value of this expression. Statement 2 is NOT sufficient. Eliminate answer choice B. The correct answer choice is either C or E.
When the statements are taken together, statement 1 allows us to simplify the fraction to \(\frac{1}{x + y}\), and statement 2 tells us that \(x + y = 4\). Substituting, we find: \(\frac{1}{x + y} = \frac{1}{4}\). Together, the statements are sufficient to answer the question.
Answer: C.
General Discussion
14 Nov 2014, 12:27
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We can start off by factoring the denominators to get
1 / (x+y)(x-y) - 1 / (x+y)(x+y)
Statement 1: not sufficient
Factor to get 2y=(x+y)(x-y)
Plug into what we were given to get 1/2y - 1 / (x+y)(x+y). Not enough info.
Statement 2: not sufficient
Plug 4 in for (x+y) in the equation we were given originally to get 1/4(x-y) - 1/16. Not enough info.
Down to C and E.
Combined we know that 2y must equal 4(x-y) so we can say that 1/4(x-y) - 1/16 = 1/2y - 1/16. We still don't have a value for y and cannot get a value of the equation.
Answer E!
1 / (x+y)(x-y) - 1 / (x+y)(x+y)
Statement 1: not sufficient
Factor to get 2y=(x+y)(x-y)
Plug into what we were given to get 1/2y - 1 / (x+y)(x+y). Not enough info.
Statement 2: not sufficient
Plug 4 in for (x+y) in the equation we were given originally to get 1/4(x-y) - 1/16. Not enough info.
Down to C and E.
Combined we know that 2y must equal 4(x-y) so we can say that 1/4(x-y) - 1/16 = 1/2y - 1/16. We still don't have a value for y and cannot get a value of the equation.
Answer E!
