1 and tg are tangent functions, and the tangent function is the ratio of the opposite side to the adjacent side in a right triangle.

2. Trigonometric function is a kind of transcendental function in elementary function in mathematics. Their essence is the mapping between the set of arbitrary angles and a set of ratio variables. The usual trigonometric function is defined in the plane rectangular coordinate system, and its domain is the whole real number domain. The other is defined in a right triangle, but it is incomplete. Modern mathematics describes them as the limit of infinite sequence and the solution of differential equation, and extends their definitions to complex system.

3. Because of the periodicity of trigonometric function, it has no inverse function in the sense of single-valued function.