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So the question posed in the book I'm working through (Book of Proof by Richard Hammack) is as follows:

Is the set $\theta=\{((x,y),(3y,2x,x+y)):x,y\in\mathbb R\}$ a function? If so, then what is its domain, codomain and range?

My answer:

Yes, this is a function since each ordered pair is distinct, thus each input value is distinct.

Domain: $\{(x,y):x,y\in\mathbb{R}\}$

Codomain: $\{(x,y,z):x,y,z\in\mathbb{R}\}$

-- Is this portion of my answer correct? And is there an easy way to determine the range of this function?

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Hint: the mapping $(x,y) \mapsto (3y,2x,x+y)$ is linear, thus the range of $\theta$ is a subspace of $\mathbb{R}^3$. By finding the matrix associated to $\theta$ you can find a basis of this subspace.

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