片段一:出示主题图。师:在体育课上,同学们正在开展各种体育活动,看了这幅图你能提出什么加法问题?生:跳绳的有多少人?生:女生有多少人?生:一共有多少人?……片段二:教学加法的结合律。1.列式计算,得出等式。师:我们来研究第三个问题,应该怎样列式?生:可以先求跳绳的有多少人,再求一共有多少人。列式是:(28+17)+23=68。生:还可以先求女生有多少人,再求总人数。列式是:28+(17+23)=68。 Fragment one: Show theme map. Teacher: In physical education class, the students are carrying out various sports activities. If you read this picture, what admissible addition problems can you give? Health: How many people are skipping? Health: How many girls are there? Health: How many people are there? ... Fragment 2: The law of combination of teaching addition. 1. Column calculation, the equation. Teacher: We should study the third question, what kind of formula should be born: you can ask how many skipping people, ask for a total of how many people. The formula is: (28 + 17) + 23 = 68. Students: You can also ask how many girls first, and then seek the total number. The formula is: 28 + (17 + 23) = 68.