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bobdylan: Senior | Next Rank: 100 Posts; Posts: 68; Joined: Wed Nov 30, 2011 3:12 pm

by bobdylan » Sat Jun 02, 2012 7:46 am

( x+y) ^2 - ( x-y)^2 = 20, then xy

a) 0
b) 1
c) 2
d) 5
e) 0

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Source: Beat The GMAT — Problem Solving | Sat Jun 02, 2012 7:46 am

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sat Jun 02, 2012 7:55 am

(x + y)Â² = (xÂ² + 2xy + yÂ²)
(x - y)Â² = (xÂ² - 2xy + yÂ²)

Hence, (x + y)Â² - (x - y)Â² = (xÂ² + 2xy + yÂ²) - (xÂ² - 2xy + yÂ²) = 4xy

Therefore, 4xy = 20 --> xy = 20/4 = 5

The correct answer is D.

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