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by Anurag@Gurome » Sat Jun 02, 2012 7:55 am
(x + y)² = (x² + 2xy + y²)
(x - y)² = (x² - 2xy + y²)

Hence, (x + y)² - (x - y)² = (x² + 2xy + y²) - (x² - 2xy + y²) = 4xy

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

The correct answer is D.
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