riables. In the selection of symbols to solve a particular problem, we must distinguish between constants and variables specific to the problem at hand. Consider the volume of a circular cone where we keep its height constant (h). The formula for its volume is V = r2h/3, where r is the radius of the base circle. H, of course, is constant for this problem, and just for this problem alone. r is the independent variable and V is the dependent variable. h is referred to as a parameter. B.2.2 The Function Concept We remember the equation for power: P = I2R. Let R be 75 S2, then P = 7512. When the value of I is known, we can calculate the power. For example, when I = 2 A, the power will be P =75 x2x2=300W; that is, the variable P depends on the variable I. In general, we can say that if a variable y depends on another variable X so that for each value of X (in a suitable set), a corresponding value of y is determined. y then is called a function of X. In symbols, this is written as y=f(x) Read this as "y is the function of x" or just "y equals f of x." Remember here that f is not a qualitative symbol but an operative symbol. Take the conversion formula from absolute temperature to centigrade: T = 273 + C. Here T = f(C). Another example is noise power. It is a function of absolute temperature and bandwidth. Thus, Pn = -228.6 dBW + 10 log T + 10 log BHz. This is Eq. (9.12) from the text. Note the use of an absolute constant-in this case, Boltz- mann's constant. Pn is the independent variable; T is the noise temperature in kelvins; and B, the bandwidth in hertz, are dependent variables. We could set the bandwidth at 1 MHz. Then Pn = (T). B.2.3 Using the Sigma Notation Greek letter (capital sigma) indicates summation. The index of summation limits the number of items to be summed or added up. The letter i is often used for this purpose. Values for i are placed above and below sigma. The initial value is placed below, and the final value is placed above the letter sigma. For example, we could have Here the initial value is 