In a probability density function, the ordinate of the random variable x represents the probability density and not the probability itself. The probability is represented by the area under the curve, so the probability of x falling between A and B is the area under the curve between x¼A and x¼B, and the probability of the random variable being equal to a certain value exactly is zero because the area of a line is zero. The area under the entire curve is, of course, equal to 1. The probability density function can usually be represented by a mathematical expression, for example,