Q. 145.0( 1 Vote )

If A = {1, 2, 3}, show that a one – one function f : A A must be onto.

Answer :

TIP: One – One Function: – A function is said to be a one – one functions or an injection if different elements of A have different images in B.


So, is One – One function


a≠b


f(a)≠f(b) for all


f(a) = f(b)


a = b for all


Onto Function: – A function is said to be a onto function or surjection if every element of A i.e, if f(A) = B or range of f is the co – domain of f.


So, is Surjection iff for each , there exists such that f(a) = b


Now, f: A A where A = {1, 2, 3} and its a One – One function


To Prove: – A is Onto function


Since it is given that f is a One – One function,


Three elements of A = {1, 2, 3} must be taken to 3 different elements of co – domain A = {1, 2, 3} under f.


Thus by definition of Onto Function


f has to be Onto function.


Hence Proved


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