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Re: If xy = 7 and x – y = 7, then x² +
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22 Mar 2017, 14:43
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GMATPrepNow wrote:
If xy = 7 and x – y = 7, then x² + y² =
(A) 35 (B) 42 (C) 49 (D) 56 (E) 63
The important thing to recognize here is that if we square (x - y), we get a x² term and a y² term. We also get an extra xy term, but that's okay, because we're given some information about the product xy.
So, take: x – y = 7 and... Square both sides: (x - y)² = 7² Expand: x² - 2xy + y² = 49 Since we're told xy = 7, we'll replace xy with 7 to get: x² - 2(7) + y² = 49 Simplify: x² - 14 + y² = 49 Add 14 to both sides: x² + y² = 63
Answer: E
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Re: If xy = 7 and x – y = 7, then x² +
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16 Jul 2017, 13:09
If xy = 7 and x – y = 7, then x² + y² =
\((x - y)^2 = x^2 - 2xy + y^2\)
\(x^2 + y^2 = (x - y)^2 + 2xy\)
\(x^2 + y^2 = (7)^2 + 2 * 7\)
\(x^2 + y^2 = 49 + 14\)
\(x^2 + y^2 = 63\)
Hence, Answer is E
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Re: If xy = 7 and x – y = 7, then x² +
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23 Aug 2018, 11:44
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