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- Feb 14, 2012

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I've found an interesting and difficult (at least for me) problem which asks us to find the minimum value of $xy$ and no matter how serious that I tried, I couldn't get myself out of the misery of finding what the question wanted. Could anyone help?

Problem:

Find the minimum value of $xy$, given that $x^2+y^2+z^2=7$, $xy+xz+yz=4$, and $x, y$ and $z$ are real numbers.