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MathNet 2015-part-07 — 185 problems (EN–VI) Question 59

Math Olympiad Vault · MathNet 2015-part-07 — 185 problems (EN–VI)

Question 59

Consider the function $z(x, y)$ describing the paraboloid
$$
z = (2x - y)^2 - 2y^2 - 3y.
$$
Archimedes and Brahmagupta are playing a game. Archimedes first chooses $x$. Afterwards, Brahmagupta chooses $y$. Archimedes wishes to minimize $z$ while Brahmagupta wishes to maximize $z$. Assuming that Brahmagupta will play optimally, what value of $x$ should Archimedes choose?