3.3 Review of probability theory 。Random variable A random variable is a variable whose values vary randomly and cannot be predicted precisely in advance 。Probability distribution function(分布函数) The probability distribution function of a random variable is probability for the random varible being smaller than or equal to a variable Fx(x)=P(X≤x)l Random variable A random variable is a variable whose values vary randomly and cannot be predicted precisely in advance l Probability distribution function(分布函数) The probability distribution function of a random variable is probability for the random varible being smaller than or equal to a variable F (x) P(X x) X  