Definition:Proportion/Joint

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Definition

Two real variables $x$ and $y$ are jointly proportional to a third real variable $z$ if and only if the product of $x$ and $y$ is a constant multiple of $z$:

$\forall x, y \in \R: x y \propto z \iff \exists k \in \R, k \ne 0: x y = k z$


Sources