Domain of a relation is the set of all its first element or x - coordintes.



Range of a relation is the set of all its second element or y-coordinates.



example 



          x       y

         -1       0

          0      1.5

          1       3



Domain: {-1, 0, 1}

Range: {0, 1.5, 3}





Function is a relation in which each element of the domain is paired with 

EXACTLY one element of the range.



        example



           x      y

           0     -1

           2     -2 

           1     -1

           1      5 

          -3      4



This is not a function because the domain 1 is paired with two ranges -1 and 5.



A function should be



          x      y

          1      7

          2      7

          3      8



even if two domain has one element of y. 

domain is paired with exactly one range 

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