a reference to symbols used in calculus (a form of mathematics dealing with things in a state of change, such as the rate of growth of an individual, the acceleration and deceleration of a bullet and the area of curved figures). The symbol d means an element of or more simply, a little bit of, and y and x are variables used to represent quantities, etc. Thus, dx means a little bit of x and dy means a little bit of y. In resolving problems with calculus, these symbols are used to express the elements of an overall whole represented by the variable x or y. A long "S" (??) means the sum of, and is often used before dy or dx to mean the sum of all the little bits that make up the whole, as in ???dy.