If xy = 7 and x – y = 7, then x² + y² =
(A) 35
(B) 42
(C) 49
(D) 56
(E) 63
Answer: E
Source: GMAT Prep Now
If xy = 7 and x - y = 7, then
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We know that (x – y)^2 = x ^2 + y^2 – 2xyBTGModeratorVI wrote: ↑Thu Jul 23, 2020 6:34 amIf xy = 7 and x – y = 7, then x² + y² =
(A) 35
(B) 42
(C) 49
(D) 56
(E) 63
Answer: E
Source: GMAT Prep Now
Thus, 7^2 = (x ^2 + y^2) – 2*7
=> x ^2 + y^2 = 63
Correct answer: [spoiler][/spoiler]
Hope this helps!
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Last edited by Jay@ManhattanReview on Mon Aug 03, 2020 9:24 pm, edited 1 time in total.
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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.BTGModeratorVI wrote: ↑Thu Jul 23, 2020 6:34 amIf xy = 7 and x – y = 7, then x² + y² =
(A) 35
(B) 42
(C) 49
(D) 56
(E) 63
Answer: E
Source: GMAT Prep Now
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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xy = 7, and x - y = 7
we know that:
(x-y)^2 = x^2 + y^2 -2xy
7^2 = x^2 + y^2 - 2x7
x^2 + y^2 = 63.
we know that:
(x-y)^2 = x^2 + y^2 -2xy
7^2 = x^2 + y^2 - 2x7
x^2 + y^2 = 63.
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Solution:BTGModeratorVI wrote: ↑Thu Jul 23, 2020 6:34 amIf xy = 7 and x – y = 7, then x² + y² =
(A) 35
(B) 42
(C) 49
(D) 56
(E) 63
Answer: E
Since x - y = 7, we have:
(x - y)^2 = 7^2
x^2 + y^2 - 2xy = 49
x^2 + y^2 - 2(7) = 49
x^2 + y^2 = 63
Answer: E
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