The transformation law of trigonometric function expansion: when the point moves left and right, the ordinate remains unchanged and the abscissa changes. When the point moves to the right, the abscissa becomes larger, and when the point moves to the left, the abscissa becomes smaller. Trigonometric function is one of the basic elementary functions, which takes the angle (the most commonly used radian system in mathematics, the same below) as the independent variable, and the angle corresponds to the coordinates where the terminal edge of any angle intersects with the unit circle or its ratio as the dependent variable. It can also be equivalently defined as the lengths of various line segments related to the unit circle. Trigonometric function plays an important role in studying the properties of geometric shapes such as triangles and circles, and is also a basic mathematical tool for studying periodic phenomena.