At research phenomena of nature in practice man is faced with a number of different physical quantities; here includes time, length, volume, speed, weight, strength, and so on. Each of them, depending on the question, takes different values, or only one. In the first case we are dealing with a variable, and the second - a constant.
If you select a specific unit, each value can be expressed as number. Mathematics usually diverted by physical meaning considered values, it is interested only in their numerical values. mathematics deals with the space forms and quantity relations of the real world.
Can D variable be a constant?