Compulsory four mainly introduces trigonometric function, which mainly requires mastering generalized angle, angle system, surface system, basic trigonometric relation, inductive formula, trigonometric function (image and nature), sum angle, difference angle formula, double angle formula, and difference product formula. The mirror image problem of y=Asin(wx+a), sine and cosine theorem, etc. Mainly use knowledge to solve practical problems, and the knowledge points are easy to understand. There seems to be a vector problem behind it.