Relation between graphs of a function and its inverse

Relation between the value of a function at a point and the limit at a point

State the relation between work function and threshold frequency.

Relation between domain and function of composite functions

Basic of Inverse Trigonometry | Relation between inverse trigonometry and function | Domain and range of ITR |Graph of ITR

Inverse function

What is the fundamental difference between an relation and a function? Is every relation a function?