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Problem 4 - Entrance Test

If x + y = 5 and x^2 + y^2 = 17, what is the value of xy?

Correct: B

We know that (x+y)^2 = x^2 + 2xy + y^2. So 5^2 = 17 + 2xy, which means 25 = 17 + 2xy. Thus, 2xy = 8, and xy = 4.