Probability Density Function
q P = the probability that a randomly selected value of a variable X falls
between a and b.
     f(x) = the probability density function.
q The probability function has to be integrated over distinct limits to
obtain a probability.
q The probability for X to have some particular value is ZERO.
q Two important properties of the probability density function:
     (1) f(x) ¾ 0 for all x within the domain of f.
     (2)