K=0, only need to calculate the symmetry axis in the minimum positive period of trigonometric function, for example, y=sinx, symmetry axis is π/2+2kπ, symmetry axis is π/2 when k=0, and symmetry axis is 5π/2 when k= 1. The calculation of symmetry axis sometimes depends on the specific situation.

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.

In mathematical analysis, trigonometric function is also defined as the solution of infinite series or specific differential equation, which allows its value to be extended to any real value or even complex value.