price may be a function of supply and demand. In general, the cost or profit depends upon a number of independent variables, for example, prices of raw materials, wages on labour, market conditions and so on. Thus a dependent variable y depends on a number of independent variables x 1, x 2, x 3 ..x n . It is denoted by y = f (x 1, x 2, x 3..xn) and is called a function of n variables. In this section, we will restrict the study to functions of two or three variables and their derivatives only.