What kind of topic is this? It's modern and probabilistic, unlike the college final exam. These topics are not difficult at first glance, just like the examples in the book.

I studied for a long time and forgot. Say it briefly.

Question 2 1: A is a concrete number matrix, which can be directly used for inversion through transformation.

Question 22: To judge whether the number matrix is related or not and the rank of the matrix, of course, the first choice is to find the rank through elementary row transformation, which is also three four-dimensional vectors. You can judge whether it is directly related according to the rank obtained by elementary transformation. If the rank is 3, it is linearly independent, otherwise it is linearly related.

Question 23: Like the previous two questions, if there is a non-zero solution in the direct column-column transformation, the rank of the matrix must be less than 3, that is, the value of the determinant is zero, from which the equation can be obtained and A can be obtained.

Question 24: Simple probability problem, directly multiply the corresponding six numbers by two and then add them to get the answer. The idea is: there are three ways to randomly select one. If it is drawn to Workshop A, the probability is 0.25, and the defective rate of Workshop A is 0.5, then the probability that the products drawn from Workshop A are still defective is 0.25*0.5=0. 125, and the other three are similar. Then add these three numbers together.

Question 25: Simple proof problem. The characteristic of unbiased estimator is that the expectation of estimator is the same as the original estimator. So the meaning of this question is: prove that U is an unbiased estimate of U, that is, prove that E (U) = E (U), and according to the expected property, E(ax+by)=aE(x)+bE(y), so you can prove it directly, which is equivalent to letting you prove 1/4+.