When preparing lessons before the lecture, I think the content of this lesson is very simple. I reviewed an example and the content of Unit 4. But when I really taught, I realized it wasn't like that. First, divide the scores of different denominators. The general fraction will inevitably find out the least common denominator, that is, the least common multiple of the denominator, and then use the basic properties of the fraction to turn all the fractions into fractions with the same denominator. In this process, a series of knowledge needs students to show it from the beginning and then do it step by step. Secondly, after becoming the same denominator score, you should add and subtract the same denominator score with what you learned in grade three, with the denominator unchanged and the numerator added and subtracted. Finally, it depends on whether the result is the simplest score. So it seems that there is only one example in a class, but there are many knowledge points involved. It also shows that the consistency of learning knowledge in mathematics content requires students to learn bit by bit and slowly, and the whole problem may not be started. This also tells us that as a teacher, we should review and summarize in time during the lecture, instead of rushing forward blindly, we should consider the students' acceptance and mastery.