In order to set the stage for a definition of combined variation, we will first discuss some of the more basic types of variation. A quantity y is said to vary directly as x if y=kx, where k is a constant, called the constant of variation. In this case, we also say that y is directly proportional to x; so we also call k the constant of proportionality. This means that as x gets larger, so does y; and as x gets smaller, so does y. We may also have y varying directly as some power of x. For example, if y=kx², then we would say that y varies directly as the square of x. If y=k/x, where k is a constant, we say that y varies inversely as x, or that y is inversely proportional to x. If y varies inversely...