1. Trigonometric function Pinyin: [nji?oh×nsh?].] Three sides of a right triangle can form six ratios about any acute angle, which are called sine and cosine of this angle; Tangent and cotangent; Secant and cotangent.

2. The trigonometric function takes the angle (the most commonly used radian system in mathematics, the same below) as the independent variable, and the coordinate of the intersection point between the terminal edge of any angle and 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. Common trigonometric functions are sine function, cosine function and tangent function.

3. Other trigonometric functions, such as cotangent function, secant function, cotangent function, orthovector function, cofactor function, semiorthovector function and semifactorial function, will be used in other disciplines, such as navigation, surveying and engineering. The relationship between different trigonometric functions can be obtained by geometric intuition or calculation, which is called trigonometric identity. Trigonometric functions are generally used to calculate the sides and angles of triangles with unknown lengths, and are widely used in navigation, engineering and physics.