Trigonometric function is a common function about angle in mathematics. In other words, a function with angle as the independent variable and the ratio of angle to any two sides as the dependent variable is called trigonometric function, which relates the inner angle of a right triangle to the ratio of the lengths of its two sides, and 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.

In a right triangle, when the connecting lines AB, AC and BC of three points A, B and C on the plane form a right triangle, where ∠ACB is a right angle. For ∠BAC, for the opposite) a=BC, hypotenuse) c=AB, and adjacent) b=AC, there is the following relationship: