Knowing the sum and difference of two numbers (generally: large number-decimal number), the application problem of finding the numbers of these two numbers is called the sum-difference application problem, which is called the sum-difference problem for short. The law of solving the sum-difference problem is: the difference between decimal and two numbers is a large number, and the difference between the sum of two numbers and two numbers is twice that of a large number; The difference between a large number and two numbers is a decimal, and the difference between two numbers and two numbers is twice the decimal. So, add the difference between two numbers to the sum of two numbers, and then divide by 2, you can find that big number; Subtract the difference between two numbers from the sum of two numbers, and then divide by 2, and you can find the decimal.

basic recipe

(sum and difference) ÷2= decimal

(sum+difference) ÷2= decimal

Case analysis

Dad bought 40 arithmetic books and 40 Chinese books. It is well known that there are four more arithmetic books than Chinese books. How many arithmetic books and Chinese books did Dad buy?

Analysis: I have been emphasizing drawing and solving problems with pictures a few days ago. Can you draw a picture on this question? But of course we can't draw 30 books. For this sum-difference problem, we can use line segments instead. For example, we can draw a line segment instead of the number of Chinese books. We also use line segments to represent the number of arithmetic books. For comparison, we align the left ends of the two line segments and draw them up and down. And mark small triangles on equal line segments.

When drawing a picture, we'd better draw all the information into a picture. At best, we don't need to look back at the topic.

Let's look at the picture. There are 40 arithmetic books and Chinese books, and there are 4 more arithmetic books than Chinese books. Does that mean that if we take away four arithmetic books, there will be as many Chinese books as arithmetic books? Then at this time, the sum of the two books will be less than four. Because there are as many arithmetic books as Chinese books after taking them away. Therefore, the number of Chinese books is

Chinese version: (40-4) ÷ 2 = 18 (this version)

So the number of math books is

Math book: 18+4=22 (this book)