“道而弗牵,强而弗抑,开而弗达”(《礼记·学记》)所指的教育理念在数学新课程改革中得到了新的注解和发展.以学生为主体,尊重学生已有的知识经验,让学生有机会进行自主的探究活动,经历知识的发生发展过程,已成为教师设计和实施教学的重要原则.本文记录了笔者“两角差的余弦公式的证明”一课的教学过程,并对情境引入与学生活动过程中的若干问题进行了反思,供同行参考.不当之处,敬请指正. The educational concept referred to by “The Road to the Evangelism, the Strong and the Fluctuation, and the Open and the Fuda” (“Book of Rites and Learning”) has received new annotations and developments in the reform of the new mathematics curriculum. Respecting students’ existing knowledge and experience, giving students the opportunity to conduct independent inquiry activities, and experiencing the process of knowledge development has become an important principle for teachers to design and implement teaching. This paper records the author’s cosine formula of the two angle difference. Prove "The teaching process of a lesson, and reflect on some problems in the introduction of the situation and student activities, for peer reference. Improper, please correct me.