正如一个学校中应有校长,一个班级中应有班长,一个家庭中应有家长,……,对于三角函数的大家庭中(正弦、余弦、正切、余切、正割、余割共六种)也应推选一个“老大”,经过深思熟虑,推选“cos”为老大,因为有以下十个方面理由: 1.高中代数上册第三章一开始就先推导cos(α+β)=cosαcosβ-sinαsinβ,由此拉开了“和、差、倍、半、和积互化、万能公式”等公式推导的帷幕,故起核心地位的作用. Just as there should be a principal in a school, there should be a squad leader in one class, a parent in a family, ..., for the trigonometric functions of the extended family (sine, cosine, tangent, cotangent, secant, co-cut, etc. ) should also be elected a “boss”, after careful consideration, elected “cos” as the boss, because there are ten reasons: 1. High school algebra on the volume of the third chapter is the beginning of the first derivation of cos(α+β)=cosαcosβ-sinαsinβ This opened the curtain of formula derivation of “harmony, difference, times, half, product and product intergration, universal formula”, and thus played a central role.