Simple math problems for junior high school, math problems for junior high school

There are two points C and D on the line AB, and the point C divides the line AB into two parts: AC: BC=2:3, and the point D divides the line AB into two parts: AD: DB=2:1 and CD=4(cm), and the length of the line segment AB is found.

AC accounts for 2 5 of AB

AD accounts for 2 3 of AB

CD accounts for 2 3-2 5 = 4 15 of AB

ab = 4 4 15 = 15 cm.

Both of these may be rational numbers or irrational numbers.

For example, the first one is: the cycle is 21211211121111211111 is a rational number, and the continuation is 211111121111111....Not really.

The second example is: the cycle time is 21...21..21 is a rational number, and if it is not, it is an irrational number.

Summary. 3x+2y 14 2x-3y -8 2 3 6x+4y 28 6x-9y -24 - 13y 52 to get y 4, x 2

Hello, please send me the math problem.

3x+2y 14 2x-3y -8 2 3 6x+4y 28 6x-9y -24 - 13y 52 x 4, x noisy pure 2

The last group can be expressed in the algebraic form of a as: 14-a people.

Add the two equations to give 2(x+y)=1-m+2>0

So m<3

Broadness and representativeness are contradictory and unified.

This will help you understand that the broadest extensiveness is the whole object of study, and one does not fall into the most extensive comprehensive investigation;

The best representation is that you take only one sample and the smallest sample study, which can be as close as possible to the actual situation of all the subjects studied.

Practical problems are examples of the height of middle school students in Beijing.

1. Extensiveness: It is not broad enough to choose a school or a district; However, selecting the middle of the 18 districts and counties, such as the suburbs (there are suburbs and urban areas), Tongzhou, or middle schools with the most students, can offset the lack of breadth and make the data more representative.

2. Representativeness: The best representativeness takes into account the extensiveness, selects 5 districts and counties, selects 4 schools in each district and county, selects 3 classes in each school, and randomly selects 10 students in each class (generally to the class can only explain the problem), and when considering the districts and counties, schools, and classes, take into account various types such as economic development, children of migrant workers in rural areas and towns, and ordinary classes in experimental classes, so that the survey is wide and scattered and the most representative.

Extensiveness: Involving a wide range, not limited, not restricted, not restricted.

Representative: representative.

For example, the investigation of China's household income should involve the whole country, and it should not go to extremes, but should be universal.

To put it simply, extensiveness means that the scope of the investigation should be wide. Representativeness means to be more popular. Although some data are extensive and surveyed, they are all selected to the extreme, so they are not representative.

For example, if we want to investigate the number of Chinese now, then we must go to various provinces and localities to sample, and this national sampling is extensive. And the samples drawn are representative, which is representativeness. I don't know how to continue asking.

There are x copies of a $4 notebook and a $7 notebook of Y. Rule.

4x+7y=84

By the title, x,y should be integers.

So {x=14 or {x=7

y=4 y=8

14-4=10 (Ben).

8-7 = 1 (Ben).

A: There may be a difference of 10 or 1 between the two notebooks.

Set up to buy 4 yuan x book, 7 yuan y book, from the title.

4x+7y=84

Because xy is an integer, y1=4, x1=, x=7 so the two notebooks may differ by 10 or 1.

In fact, this question is not difficult, but the process is more difficult to understand and difficult to find.

Let the 4 yuan notebook x, the 7 yuan notebook y, 4x+7y=84, when x=7, y=8; x=14,y=4

So there may be a difference of 1 or 10 between the two notebooks.

Solution: (1) Set up a factory to purchase x tons of raw materials from place A and make Y tons of products shipped to place B

12- gets: 13y=3900, solves: y=300, substitutes y=300 into gets: x=400, and the set of solutions for the system of equations is:

x=400y=300​

After testing that x=400 and y=300 are in line with the topic, the factory purchased 400 tons of raw materials from place A and made 300 tons of products shipped to place B;

2) According to the topic: 300 8000-400 1000-15000-97200 = 1887800 (yuan), the sales of this batch of products are 1887800 yuan more than the sum of raw material costs and transportation costs

Hopefully, thank you!

1.Let x be the raw material and y be the finished product. Rule.

120*,10*,x=400,y=300,2Sales 8000 * 300 = 2400000, 1000 * 400 + 15000 + 97200 = 512200, so there are more than 2400000-512200 = 1887800 hopes.