The function \( f(x, y) = 9 - x^2 - 9y^2 \) is a quadratic function in two variables. It can be rewritten as \( f(x, y) = 9 - x^2 - 9y^2 \), which is in the form of an elliptic paraboloid. This is because it has the general form \( z = c - ax^2 - by^2 \), where
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