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## prettydoc TRUE
Give the domain and range of the multivariable function.
- \(f(x, y) = x + 2y\)
domain: \(\mathbb{R}^2\)
range: \(\mathbb{R}\)
- \(f(x, y) = \frac{1}{\sqrt{x^2 + y^2 - 9}}\)
domain: \(\{(x, y)|x^2 + y^2 > 9\}\) , i.e., the domain lies outside of a circle centered at the origin with radius 3.
range: \(z > 0\)