三角函数的理论来源于几何,用几何定义了三角函数,并用代数方法研究函数,建立了三角形中各元素的种种关系,它不仅能解决由已知元素求未知元素的问题,也为用三角法证明几何题奠定了基础.1　应用三角函数的定义　　如果待证的题目与直角三角形有关,或者可以转化为直角三角形的问 The theory of trigonometric functions comes from geometry, geometrically defines trigonometric functions, and uses algebraic methods to study functions and establishes relationships between elements in a triangle. It not only solves the problem of finding unknown elements from known elements, but also uses trigonometry. Proof of the geometric problem lays the foundation.1 Applying the definition of a trigonometric function If the subject to be proved is related to a right-angled triangle, or can be converted to a right-angled triangle