First, the main points of knowledge

1. formulas of trigonometric functions's deformation should start from the following three aspects by using the trigonometric formula:

(1) Renaming: Pay attention to the differences and connections between the names of trigonometric functions in conditions and conclusions, and achieve the purpose of unifying the names of trigonometric functions through formulas, inductive formulas and universal formulas.

(2) Angle change: Pay attention to the difference and connection between the conditions of trigonometric function and the angle in the conclusion, and unify the angle of trigonometric function through inductive formula and formulas of trigonometric functions of sum, difference, multiple and half angle.

(3) Variable operation form: formalize the operation of conditions and conclusions as needed, formalize the operation of one side of the equation into the operation form of the other side, and achieve the goal through the transformation of elevation.

2. Trigonometric functions in triangles (interior angle sum theorem, sine theorem, cosine theorem)

3. When applying the trigonometric transformation formula, we should pay attention to the relationship between the formulas and the conditions under which the formulas are established. The structural characteristics of each triangular formula determine its two-way function, and from left to right and from right to left can often play different roles. The so-called triangle identity deformation means that there is an identity relationship under meaningful conditions, but the triangle transformation often changes the range of the angle in the triangle. Therefore, when discussing the function properties of formulas of trigonometric functions representation, we must first determine its domain.