余弦定理和正弦定理是中学数学中的重要内容之一 ,两者可互为依据 ,相互推导 .随着学生学习的深入 ,知识面的扩大 ,抽象思维能力的提高 ,可进一步从不同的角度揭示二者的关系 ,加强应用 .余弦定理 :在△ ABC中 ,三边 a,b,c和它们所对的角∠ A,∠ B,∠ C之间有如下关系 :a2 =b2 Cosine theorem and sine theorem are one of the important contents of mathematics in middle school, which can be mutually based on each other and deduced from each other.With the deepening of students' learning, the expansion of knowledge and the improvement of abstract thinking ability, it can be further revealed from different angles The relationship between the two, to strengthen the application. Cosine theorem: △ ABC, the three sides a, b, c and their angle between the corners A, ∠ B, ∠ C has the following relationship: a2 = b2