Trigonometric function is the most changeable part in traditional knowledge. When dealing with this part of the content, the new textbook has an obvious tendency of falling tone, highlighting the role of "sum, difference and angle-doubling formula", highlighting the main position of sine and cosine functions, and strengthening the examination of the image and nature of trigonometric functions. Therefore, the nature of trigonometric function is the focus of this chapter review. The first round of review should focus on the reproduction of textbook knowledge, pay attention to the implementation of basic knowledge points, re-understand basic methods and master basic skills, and strive to be systematic, organized and networked to form a relatively complete knowledge system; The second and third rounds of review are mainly based on the basic comprehensive test, and the comprehensive test questions should be close to the college entrance examination questions in form, but not difficult. Of course, this part of knowledge is most likely to appear in the proposition of "combining with reality, using a little trigonometric transformation (especially the application of cosine double angle formula and formula in special cases) to investigate the properties of trigonometric functions", and the difficulty is to master the variant application of cosine double angle formula flexibly. Because the triangle problem is a basic problem and a routine problem, it belongs to the category of easy problems. Therefore, it is suggested that the review of trigonometric functions should be controlled within the scope and difficulty of textbook knowledge in order to adapt to the trend of future college entrance examination propositions. In short, the review of trigonometric function should be based on the foundation, strengthen training, comprehensive application and improve ability.

General strategies to solve the triangle problem of college entrance examination;

(1) Find the difference: observe the difference between the angle and the function operation, that is, conduct the so-called "difference analysis".

(2) Find the connection: use the relevant triangle formula to find out the internal connection between the differences.

(3) Reasonable transformation: choose the appropriate triangle formula to promote the transformation of differences.

Basic strategies for constant deformation of trigonometric functions;

(1) constant substitution: especially "1" substitution, such as 1=cos2θ+sin2θ=tanx? Cotx=tan45, etc.

(2) Division of items and matching of angles. Such as split term: sin2x+2cos2x = (sin2x+cos2x)+cos2x =1+cos2x; Matching angle: α = (α+β)-β, etc.

(3) Degeneracy, that is, the degeneracy of the double-angle formula.

(4) Chord (tangent) method. Using the basic relationship of trigonometric function with the same angle, the trigonometric function is transformed into a chord (tangent).

(5) Introduce an auxiliary angle.