GMAT Problem Solving (PS) | Re: If x^2 + y^2 = xy, then (x + y)^4 =

Given: x^2 + y^2 = xy
If we add 2xy to both sides, we get: x^2 + y^2 + 2xy = 3xy Or (x+y)^2 = 3xy

Now, (x+y)^4 = [(x+y)^2]^2 = (3xy)^2 = 9 * x^2 * y^2

Hence option C

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