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Let $x$ and $y$ be complex numbers. If $x + y =2$ and $x^3 + y^3 = 5$, then what is $x^2 + y^2$?

 Feb 11, 2024
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Our current goal is to find the value of xy, so we can get x^2 + y^2 = (x + y)^2 - 2xy.

x^3 + y^3 can be factored to (x + y)(x^2 - xy + y^2)

Additionally, x^2 - xy + y^2 = (x + y)^2 - 3xy 

So, x^3 + y^3 = (x + y)(x^2 - xy + y^2) = (x + y)[(x + y)^2 - 3xy], now substitute:

2 * (4 - 3xy) = 5  => 4 - 3xy = 2.5 => 1.5 = 3xy => xy = 0.5

Now, to get x^2 + y^2, just do (x + y)^2 - 2xy = 4 - 1 = 3

 Feb 11, 2024

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