Q. 123.8( 8 Votes )

Let R be the relation defined on the set A = {1, 2, 3, 4, 5, 6, 7} by R = {(a, b) : both a and b are either odd or even}. Show that R is an equivalence relation. Further, show that all the elements of the subset {1, 3, 5, 7} are related to each other, and all the elements of the subset {2, 4, 6} are related to each other, but no element of the subset {1, 3, 5, 7} is related to any element of the subset {2, 4, 6}.

Answer :

Given A = {1, 2, 3, 4, 5, 6, 7} and R = {(a, b) : both a and b are either odd or even number}

Therefore,


R = {(1, 1), (1, 3), (1, 5), (1, 7), (3, 3), (3, 5), (3, 7), (5, 5), (5, 7), (7, 7), (7, 5), (7, 3), (5, 3), (6, 1), (5, 1), (3, 1), (2, 2), (2, 4), (2, 6), (4, 4), (4, 6), (6, 6), (6, 4), (6, 2), (4, 2)}


To prove that relation is equivalence, we need to prove that it is reflexive, symmetric and transitive.


Reflexivity : For Reflexivity, we need to prove that-


(a, a) R


Here (1,1), (2,2), (3,3), (4,4), (5,5), (6,6), (7,7) all R


From the relation R it is seen that R is reflexive.


Symmetric: For Symmetric, we need to prove that-


If (a, b) R, then (b, a) R


From the relation R, it is seen that R is symmetric.


Transitive: For Transitivity, we need to prove that-


If (a, b) R and (b, c) R, then (a, c) R


[I (a, b) are odd and (b, c) are odd then (a, c) are also odd numbers]


From the relation R, it is seen that R is transitive too.


Also, from the relation R, it is seen that {1, 3, 5, 7} are related with each other only and {2, 4, 6} are related with each other .


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