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Problem 3 - Entrance Test

If (x + y)^2 = 16 and (x - y)^2 = 4, what is the value of xy?

Correct: B

(x + y)^2 = x^2 + 2xy + y^2 = 16 and (x - y)^2 = x^2 - 2xy + y^2 = 4. Subtracting the second equation from the first gives: (x^2 + 2xy + y^2) - (x^2 - 2xy + y^2) = 16 - 4 => 4xy = 12 => xy = 3.