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一、“思”是排列还是组合求概率求离散型随机变量的数学期望,首先要求好离散型随机变量的概率值,而求离散型随机变量的概率,就要设计用好排列与组合这一解题工具.排列与组合问题最值得我们思量的问题之一是元素有无顺序的问题.这一问题决定我们是用排列还是用组合解决问题,从而决定了结论的正确与否.实例1:无放回的方式从袋中取球,取出球无顺序要求用组合例1一个袋中装有3个白球和2个黑球,它
First, “thinking ” is arranged or combination of seeking the mathematical probability of seeking discrete random variables, first of all require good discrete random variable probability value, and seek the probability of discrete random variables, it is necessary to design a good arrangement and combination The solution to the problem Arrangement and combination problems One of the most worthy questions that we think about is the question of the order of the elements.The question determines whether we use permutations or combinations to solve the problem and thus determine whether the conclusion is correct or not. 1: no way to get back the ball from the bag, remove the ball No order required with the combination of Example 1 a bag contains three white balls and two black balls, it