Teaching begins in the third grade. According to the inquiry of Shanghai junior high school mathematics textbook, trigonometric function is taught in grade three. In the first semester of Grade Three, I study acute trigonometric functions (sine, cosine and tangent), and in the second semester of Grade Three, I study obtuse trigonometric functions (sum-difference product, product-sum-difference, half-angle formula). Trigonometric function is one of the basic elementary functions in mathematics. It takes angle (the most commonly used radian system in mathematics) as the independent variable, and the number of directed line segments from the position on the terminal edge of any angle to the origin as the dependent variable.