在概率与统计学习中,我们遇到这样一个问题:有20件产品,其中有5件次品,不放回的抽取3件,则其中含次品数的期望为.由ξ分布列,我们可得期望为0.75,而有的同学把题看错了,按放回式抽样用二项分布计算,我们知道书本上有习题指出,数量很大的产品,结果才符合二项分布Eξ=nP.而在这里,n=20,它们期望 In the study of probability and statistics, we have encountered such a problem: There are 20 products, of which there are 5 defective products, 3 products are not put back, and the expectation of the number of defective products is. By the distribution of ξ, we can Expected to be 0.75, and some students put the question wrong, according to the back-to-back sampling binomial distribution calculation, we know that there are exercises in the book pointed out that a large number of products, the results meet the binomial distribution E ξ = nP. And here, n=20, they expect