求离散型随机变量的期望、方差,首先要明确概率分布,最好确定随机变量概率分布的模型,这样就可以直接运用公式进行计算.不难发现,正确求出离散型随机变量的分布列是解题的关键.在求离散型随机变量的分布列之前,要弄清楚随机变量可 To find the expectation and variance of discrete random variables, we must first define the probability distribution. It is better to determine the probability distribution model of random variables, so that we can directly use the formula to calculate. It is not difficult to find that the distribution column of discrete random variables is correctly found. The key to solving the problem. Before finding the distribution of discrete random variables, be aware of the random variables