In Section 1.1, we talked about constants and variables in an abstract way. When we deal with the applications of mathematics in science, engineering, etc., we can say:

- A
**quantity**is anything that can be measured, as distance, time, weight, and number.

- A quantity whose value remains unchanged throughout a given problem or discussion is called a
**constant**.

For example, if we assume that the temperature does not change during an experiment, then the temperature (often denoted by $T$ in physics) is a constant.

- A quantity which changes its value in the course of a problem or discussion is called a
**variable**.

For example, if we heat up the water in a container, its temperature $T$ changes. Then here, $T$ is a variable. Or consider a particle that moves in a circle (Figure 1). If we connect this particle to the center of the circle, the angle $\theta$ that this ray makes with the horizon is a variable that varies between 0 and $360^{\circ}$, but the distance of this particle from the center of the circle (denoted by $R$) is a constant.

or**Parameters**are constants to which infinitely many numbers may be assigned, and they retain these assigned values through the investigation.**arbitrary constants**

For example, one form of the equation of a straight line is $y=mx+b$, where $m$ and $b$ represent the slope and the $y$-intercept of the line. $m$ and $b$ can assume any values but those values are fixed numbers, so $m$ and $b$ are two parameters of the equation of a straight

line.