1.

Let Z be the set of all integers and R be the relation on Z defined as R = {(a, b): a, b ∈ Z) and (a – b) is divisible by 5. Prove that R is an equivalence relation.

Answer»

Reflexive: Since 5 divides a – a for all a ∈ Z, therefore, R is reflexive. 

Symmetric: (a, b) ∈ R ⇒ 5 divides a – b 

⇒ 5 divides b – a ⇒ b – a ∈ R 

∴ R is symmetric. 

Transitive: (a, b) ∈ R and (b, c) ∈ R 

⇒ a – b and b – c are both divisible by 5 

⇒ a – b + b – c is divisible by 5 

⇒ (a – c) is divisible by 5 

⇒ (a, c) ∈ R 

∴ R is transitive. 

Since R is reflexive, symmetric and transitive, therefore, R is an equivalence relation.



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