(A33 stands for the arrangement of 3, A55 stands for the arrangement of 5. Here I regard all letters as different, including 3 R, just calculating the probability of opposing events. )
2. "The probability that at least one of them is a red ball"
P (probability that at least one is a red ball) = 1-P (probability that none is a red ball)
= 1-( 1- 1/3)*( 1- 1/2)=2/3
This question is borrowed from others and is given in the following resources.
(1) Simple random sampling is to directly extract n elements from a population containing n elements according to the principle of equal probability (n >: n). This method is simple and easy to learn, and the common method is drawing lots. However, this applies to the case where the overall unit is small.
(2) Systematic sampling is a method of sorting the number of elements in the population, then calculating a certain interval, and then taking a fixed number of elements to form a sample. Suitable for large population and sample size. It should have a complete sampling framework, just like simple random sampling. For example, take 100 students from 3000 students, number the list of 3000 students in turn, and then take/kloc-0 every 30 students according to the formula k (sampling interval) =N (overall size) /n (sample size) =3000/ 100=30.
(3) Stratified sampling is to divide all elements into several types or levels according to certain characteristics or signs (such as age, gender, occupation or region, etc.). ), and using the first two methods to extract a sub-sample, all sub-samples constitute the total sample. For example, in a school sampling survey, people can be divided into boys and girls first, and then 65,438+000 boys and girls are selected by simple random sampling method or systematic sampling method respectively, so that these 200 students constitute a sample obtained by divided sampling.
Nowadays, high school students should learn to return. It's amazing. I didn't learn until I went to college. I don't quite understand. How far do you want me to say the regression equation? Let me talk about the basics.
Take a variable as an example: y=bx+k, we have a bunch of observations of (x, y), and we need to get the parameters b, k.
The method we use is the least square estimation method. That's more troublesome to write. I don't think high school needs to master it. I'll tell you the result.
Estimation of b =cov(x, y)/cov(x, x)
Estimation of k = average of y-average of x * estimation of b
Where cov(x, y) represents the covariance of x and y.
If you have any questions, you can call me directly. I can't seem to insert a picture here. sorry
I hope it helps you.