Definition:Proportion

Definition
Two real variables $$x$$ and $$y$$ are proportional iff one is a constant multiple of the other:
 * $$\forall x, y \in \R: x \propto y \iff \exists k \in \R, k \ne 0: x = k y$$

Inverse Proportion
Two real variables $$x$$ and $$y$$ are inversely proportional iff their product is a constant:
 * $$\forall x, y \in \R: x \propto \frac 1 y \iff \exists k \in \R, k \ne 0: x y = k$$

Joint Proportion
Two real variables $$x$$ and $$y$$ are jointly proportional to a third real variable $$z$$ iff the product of $$x$$ and $$y$$ is a constant multiple of $$z$$:
 * $$\forall x, y \in \R: x y \propto z \iff \exists k \in \R, k \ne 0: x y = k z$$

Constant of Proportionality
The constant $$k$$ is known as the constant of proportion, or (more common nowadays, but uglier) constant of proportionality.