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MathNet 2015-part-04 — 191 problems (EN–VI) Question 59

Math Olympiad Vault · MathNet 2015-part-04 — 191 problems (EN–VI)

Question 59

Let $m, n, p$ be fixed positive real numbers which satisfy $mnp = 8$. Depending on these constants, find the minimum of
$$
x^2 + y^2 + z^2 + mxy + nxz + pyz
$$
where $x, y, z$ are arbitrary positive real numbers satisfying $xyz = 8$. When is the equality attained?

a) $m = n = p = 2$
b) arbitrary (but fixed) positive real numbers $m, n, p$.