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
- 1972: George F. Simmons: Differential Equations ... (previous) ... (next): $1$: The Nature of Differential Equations: $\S 4$: Growth, Decay and Chemical Reactions: Problem $1$