
• A constant rate of change between $x$ and $y$ means that, for any fixed amount of change $\Delta x$, the corresponding amounts of change $\Delta y$ are constant.
• If $x$ and $y$ represent the measures of two quantities, and $y$ varies at a constant rate of $\dfrac {\Delta y}{\Delta x}$ with respect to x, then the change in y is always $\dfrac {\Delta y}{\Delta x}$ as much as the corresponding change in $x$.